CppEditor: Fix missing semantic highlighting

In case the editor is hidden before semantic highlighting started
(because semantic parsing hasn't finished yet).
Just disable the logic that tries to optimize that for now, since it is
not working, and fixing it will require more work (that's done later
anyhow).

Task-number: QTCREATORBUG-11367
Change-Id: I7ef3cc558e514968f90d2ea56b2df362606d879a
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/cpptools/cpptoolseditorsupport.cpp

@@ -363,11 +363,6 @@ void CppEditorSupport::startHighlighting()
     if (!m_highlightingSupport)
         return;
 
-    // Start highlighting only if the editor is or would be visible
-    // (in case another mode is active) in the edit mode.
-    if (!Core::EditorManager::visibleEditors().contains(m_textEditor))
-        return;
-
     if (m_highlightingSupport->requiresSemanticInfo()) {
         Snapshot snapshot;
         Document::Ptr doc;