Revert "Fixed 'Bug 5687 - Quick fix for extension methods must be declared static'"

This reverts commit e32d91e0653251aea26da5f17aed3bd516bff36c. This commit changes the API and is committed to the 'Retina' feature branch.

Conflicts:
	main/external/nrefactory
	main/src/addins/CSharpBinding/MonoDevelop.CSharp.Highlighting/HighlightUsagesExtension.cs