Remove the disable threading option

It was added ages ago when our threading code was not perfect, but should be top notch now so this makes not any sense

conf/dlgperformancebase.ui

@@ -52,13 +52,6 @@
             </property>
            </widget>
           </item>
-          <item>
-           <widget class="QCheckBox" name="kcfg_EnableThreading">
-            <property name="text">
-             <string>Enable &amp;background generation</string>
-            </property>
-           </widget>
-          </item>
          </layout>
         </item>
         <item>

conf/okular.kcfg

@@ -65,9 +65,6 @@
   <entry key="EnableCompositing" type="Bool" >
    <default>true</default>
   </entry>
-  <entry key="EnableThreading" type="Bool" >
-   <default>true</default>
-  </entry>
   <entry key="TextAntialias" type="Enum" >
    <default>Enabled</default>
    <choices>

core/document.cpp

@@ -2363,7 +2363,6 @@ void Document::requestPixmaps( const QLinkedList< PixmapRequest * > & requests,
     }
 
     // 2. [ADD TO STACK] add requests to stack
-    bool threadingDisabled = !Settings::enableThreading();
     QLinkedList< PixmapRequest * >::const_iterator rIt = requests.constBegin(), rEnd = requests.constEnd();
     for ( ; rIt != rEnd; ++rIt )
     {
@@ -2382,9 +2381,6 @@ void Document::requestPixmaps( const QLinkedList< PixmapRequest * > & requests,
         if ( !request->asynchronous() )
             request->d->mPriority = 0;
 
-        if ( request->asynchronous() && threadingDisabled )
-            request->d->mAsynchronous = false;
-
         // add request to the 'stack' at the right place
         if ( !request->priority() )
             // add priority zero requests to the top of the stack

doc/index.docbook

@@ -1580,14 +1580,6 @@ Add a 'Find Previous' entry
 							outline or opaque fill styles and increase speed on selections.</para>
 					</listitem>
 				</varlistentry>
-				<varlistentry>
-					<term><guilabel>Enable background generation</guilabel></term>
-					<listitem>
-						<para>Use a background thread to generate the pages. By disabling
-							this option the user interface will become less reactive (will be blocked
-							if necessary), but pages will be displayed a bit faster.</para>
-					</listitem>
-				</varlistentry>
 				<varlistentry>
 					<term><guilabel>Memory Usage</guilabel></term>
 					<listitem>

ui/pageview.cpp

@@ -4001,8 +4001,7 @@ void PageView::slotRequestVisiblePixmaps( int newValue )
 
     // if preloading is enabled, add the pages before and after in preloading
     if ( !d->visibleItems.isEmpty() &&
-         Okular::Settings::memoryLevel() != Okular::Settings::EnumMemoryLevel::Low &&
-         Okular::Settings::enableThreading() )
+         Okular::Settings::memoryLevel() != Okular::Settings::EnumMemoryLevel::Low )
     {
         // as the requests are done in the order as they appear in the list,
         // request first the next page and then the previous

ui/presentationwidget.cpp

@@ -1227,7 +1227,7 @@ void PresentationWidget::requestPixmaps()
     // restore cursor
     QApplication::restoreOverrideCursor();
     // ask for next and previous page if not in low memory usage setting
-    if ( Okular::Settings::memoryLevel() != Okular::Settings::EnumMemoryLevel::Low && Okular::Settings::enableThreading() )
+    if ( Okular::Settings::memoryLevel() != Okular::Settings::EnumMemoryLevel::Low )
     {
         int pagesToPreload = 1;