Seal and tweak 'IWinRTObject' members

[Sergio0694]

This PR tweaks some members of IWinRTObject following advice from @michalstrehovsky:

• Seal all the "Fallback" methods and other helpers we only need from CsWinRT. This will avoid an interface dispatch and will turn them into normal instance methods. Also, literally nobody should have implemented these helper methods anyway.
• Also make them all internal, since nobody should ever use them outside of CsWinRT
• Delete the leftover and not working GetOrCreateTypeHelperData, once again nobody should have ever used this.