CodeAssist: Use settable property for IAssistProposal::isFragile

Removes the need to create yet another subclass (for overriding a
virtual method) in some situations.

Change-Id: I55d70de60b4f8d127a175d996f797700c2f172a6
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/texteditor/codeassist/functionhintproposal.h

@@ -38,7 +38,6 @@ public:
     FunctionHintProposal(int cursorPos, IFunctionHintProposalModel *model);
     ~FunctionHintProposal();
 
-    bool isFragile() const override;
     IAssistProposalModel *model() const override;
     IAssistProposalWidget *createWidget() const override;