Remove xdbe support (X11)

[Albrecht-S]

Bill Spitzak (@spitzak) wrote in thread "FLTK vs NTK" in fltk.general:

It's also fine to scrap xdbe and overlay support, that is long obsolete and even when it worked it became clear that they are slower than just using X images (except on Irix which is where this was all first written).

Thanks, Bill.

This issue is about removing xdbe support only to separate issues and commits.

See also issue #254 "Remove hardware overlay support (X11)".