Editor: always configured assistant

A code assistant is not usable without a text editor widget and is
always created as part of one, so make the dependency obvious by require
it in the construction of the code assistant. This removes a bunch of
functions and cheks.

Change-Id: I96556430082ff729d99d2ae2516599f9b8cbc704
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/texteditor/codeassist/codeassistant.h

@@ -20,11 +20,9 @@ class CodeAssistant : public QObject
     Q_OBJECT
 
 public:
-    CodeAssistant();
+    CodeAssistant(TextEditorWidget *editorWidget);
     ~CodeAssistant() override;
 
-    void configure(TextEditorWidget *editorWidget);
-
     void process();
     void notifyChange();
     bool hasContext() const;