graphics/mesa-demos: Stop shipping programs that upstream don't ship

Stop thinking we are better than upstream and don't ship programs
that upstream don't

This removes :

• eglgears_x11
• eglkms (which segfaults anyway)
• egltri_x11
• peglgears (stuck in a loop)
• xeglgears
• xeglthreads (doesn't work)

Differential Revision: https://reviews.freebsd.org/D44968
Sponsored by: Beckhoff Automation GmbH & Co. KG