TextEditor: Make completion assist provider a data member

In case of the CppEditor a direct member is not possible due
to setup restrictions inside the CppEditor machinery. I'd expect
that to be fixable when the editor base system is in good shape.

Change-Id: I184e219ca2dff6f67c9b58c182212eb12972cc84
Reviewed-by: Christian Stenger <christian.stenger@digia.com>

src/plugins/cmakeprojectmanager/cmakeeditor.h

@@ -53,7 +53,6 @@ public:
     CMakeEditor(CMakeEditorWidget *);
 
     Core::IEditor *duplicate();
-    TextEditor::CompletionAssistProvider *completionAssistProvider();
 
     QString contextHelpId() const;