Remove dead code. During 8.6 alpha/beta, Tk_DefineBitmap/Tk_GetBitmapFromData moved to stub entry 274/275
due to a signature change, but later that turned out not be necessary at all. The stub entries were kept
in order not to lose binary compatibility for extensions that were compiled against beta 8.6 headers. Now that
Tk 8.6 is final, this makes no sense any more.