Remove the Emacs Lisp specific escaping test.

In fact, nuke the infrastructure altogether.

Parens as characters without a backslash are nasty and they really should
never be used:
 - This syntax is discouraged by the GNU Emacs Lisp Reference Manual.
 - They confuse many a minor mode, such as show-paren-mode.
 - The way we currently handle them partially solves the problem, but
   is a hack made of eldritch magic, implemented by gluing special cases
   together hoping they'll work...
 - ...but it doesn't really work, and it's pretty easy to get
   mishighlighted delimiters anyway.
 - They confuse the Emacs Lisp indentation engine. Really.
 - It's the major mode's job to set the syntax-table properties on
   individual characters.

Because of these reasons, it's just easier to drop this misfeature
and rely on people to start using ?\( instead.

And because the whole point of
rainbow-delimiters-escaped-char-predicate-list is to allow for this,
nuke it too. It was never *really* a documented part of the API.

rainbow-delimiters-test.el

@@ -194,17 +194,17 @@
 
 (ert-deftest doesnt-highlight-escaped-delimiters ()
   (with-temp-buffer-in-mode 'emacs-lisp-mode
-    (with-string (str "(bar ?\\( ?( (foo?))")
+    (with-string (str "(bar ?\\( (foo?))")
       (should (ert-equal-including-properties
                (buffer-string)
-               #("(bar ?\\( ?( (foo?))"
+               #("(bar ?\\( (foo?))"
                  0 1
                  (face (rainbow-delimiters-depth-1-face))
-                 12 13
+                 9 10
                  (face (rainbow-delimiters-depth-2-face))
-                 17 18
+                 14 15
                  (face (rainbow-delimiters-depth-2-face))
-                 18 19
+                 15 16
                  (face (rainbow-delimiters-depth-1-face))))))))
 
 (ert-deftest cycles-faces ()

rainbow-delimiters.el

@@ -184,27 +184,6 @@ The delimiter is not highlighted if it's a blacklisted delimiter."
                         (rainbow-delimiters--depth-face depth)))))
       (font-lock-prepend-text-property loc (1+ loc) 'face delim-face))))
 
-(defvar rainbow-delimiters-escaped-char-predicate nil)
-(make-variable-buffer-local 'rainbow-delimiters-escaped-char-predicate)
-
-(defvar rainbow-delimiters-escaped-char-predicate-list
-  '((emacs-lisp-mode . rainbow-delimiters--escaped-char-predicate-emacs-lisp)
-    (lisp-interaction-mode . rainbow-delimiters--escaped-char-predicate-emacs-lisp)
-    (inferior-emacs-lisp-mode . rainbow-delimiters--escaped-char-predicate-emacs-lisp)
-    ))
-
-(defun rainbow-delimiters--escaped-char-predicate-emacs-lisp (loc)
-  "Non-nil iff the character at LOC is escaped as per Emacs Lisp rules."
-  (and (eq (char-before loc) ?\?) ; e.g. ?) - deprecated, but people use it
-       (not (and (eq (char-before (1- loc)) ?\\) ; special case: ignore ?\?
-                 (eq (char-before (- loc 2)) ?\?)))
-       ;; Treat the ? as a quote character only when it starts a symbol, so
-       ;; we're not confused by (foo?), which is a valid function call.
-       (let ((inhibit-changing-match-data t))
-         (save-excursion
-           (goto-char (1- loc))
-           (looking-at "\\_<")))))
-
 (defun rainbow-delimiters--char-ineligible-p (loc ppss delim-syntax-code)
   "Return t if char at LOC should not be highlighted.
 PPSS is the `parse-partial-sexp' state at LOC.
@@ -228,9 +207,7 @@ Returns t if char at loc meets one of the following conditions:
     ((/= 0 (logand #x20000 delim-syntax-code))
      (/= 0 (logand #x10000 (or (car (syntax-after (1- loc))) 0))))
     (t
-     nil))
-   (when rainbow-delimiters-escaped-char-predicate
-     (funcall rainbow-delimiters-escaped-char-predicate loc))))
+     nil))))
 
 (defconst rainbow-delimiters--delim-regex "\\s(\\|\\s)"
   "Regex matching all opening and closing delimiters the mode highlights.")
@@ -240,8 +217,6 @@ Returns t if char at loc meets one of the following conditions:
   "Highlight delimiters in region between point and END.
 
 Used by font-lock for dynamic highlighting."
-  (setq rainbow-delimiters-escaped-char-predicate
-        (cdr (assoc major-mode rainbow-delimiters-escaped-char-predicate-list)))
   (let* ((inhibit-point-motion-hooks t)
          ;; Point can be anywhere in buffer; determine the nesting depth at point.
          (last-ppss-pos (point))