CodeAssist: Move base position to interface.

Every proposal needs to have a base postion.
Move it to the base class to reduce duplicated code.

Change-Id: I0b1b8282dfe955d251646661f755bf9f585fbad1
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/texteditor/codeassist/genericproposal.h

@@ -41,7 +41,6 @@ public:
     ~GenericProposal();
 
     bool isFragile() const override;
-    int basePosition() const override;
     bool isCorrective() const override;
     void makeCorrection(TextEditorWidget *editorWidget) override;
     IAssistProposalModel *model() const override;
@@ -51,7 +50,6 @@ protected:
     void moveBasePosition(int length);
 
 private:
-    int m_basePosition;
     GenericProposalModel *m_model;
 };