Don't remove syntax errors on each document change

Especially when several error markers exist, the flashing this causes
while editing isn't really nice.

Reviewed-by: Roberto Raggi

src/plugins/cpptools/cppmodelmanager.h

@@ -168,10 +168,6 @@ private:
 
     mutable QMutex mutex;
 
-    enum {
-        MAX_SELECTION_COUNT = 5
-    };
-
     struct Editor {
         QPointer<TextEditor::BaseTextEditor> widget;
         QList<QTextEdit::ExtraSelection> selections;