cmTarget: Remove vestigal method declaration.

The definition was removed in commit v3.1.0-rc1~688^2~5 (cmTarget:
Allow any generator expression in SOURCES property., 2014-03-18).

Source/cmTarget.h

@@ -614,8 +614,6 @@ private:
 
   void MaybeInvalidatePropertyCache(const std::string& prop);
 
-  void ProcessSourceExpression(std::string const& expr);
-
   // Internal representation details.
   friend class cmTargetInternals;
   friend class cmGeneratorTarget;