Remove obsolete comment about non-const references.

They used to be banned by the C++ style guide, but positions shifted, and this comment is no longer up to date.

There's another reference (heh) on lines 816-819 to how gmock is a "general framework", with the possible implication that support for non-const references was weird in some way and only there to be "general", but I left it alone because I don't really feel I understand what it's saying.

PiperOrigin-RevId: 339323428