remove an excessive mutable type specifier

MockFunction's private member cannot be reached from outside and all its
non-static member functions are not const.

googlemock/include/gmock/gmock-spec-builders.h

@@ -1870,7 +1870,7 @@ class MockFunction<R(Args...)> {
   }
 
  private:
-  mutable internal::FunctionMocker<R(Args...)> mock_;
+  internal::FunctionMocker<R(Args...)> mock_;
 };
 
 // The style guide prohibits "using" statements in a namespace scope