xsession: Disable session driver installation by default.

This may yet be moved to the kdesrc-build-setup script instead so
disable this by default so that those tracking kdesrc-build git don't
have to necessarily deal with this handling. It can still be manually
enabled.

This will cause a conflict with the 'modularize' branch, which should be
resolved by merging this change into the ksb::BuildContext Perl module.

kdesrc-build

@@ -1625,7 +1625,7 @@ EOF
         "git-repository-base"  => {}, # Base path template for use multiple times.
         "http-proxy"           => '', # Proxy server to use for HTTP.
         "install-after-build"  => 1,  # Default to true
-        "install-session-driver" => 1,# Default to true
+        "install-session-driver" => 0,# Default to false
         "kdedir"               => "$ENV{HOME}/kde",
         "kde-languages"        => "",
         "libpath"              => "",