Don't make private wxGraphicsContext-derived classes dynamically creatable.

Don't make private wxGraphicsContext-derived classes dynamically creatable.

This doesn't make any sense because classes such as wxGDIPlusContext,
wxGDIPlusMeasuringContext and wxMacCoreGraphicsContext are private and don't
need to be creatable by name (only renderers potentially need to be) and was
inconsistent with Unix/Cairo backend in which these classes were not dynamic.

So remove wxRTTI macros from them in wxMSW/wxOSX as well as this allows us to
get rid of unnecessary default ctors.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69355 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775