mesa: remove GL_MESA_program_debug extension

This was never fully fleshed out and hasn't been used.

src/mesa/SConscript

@@ -189,7 +189,6 @@ if env['platform'] != 'winddk':
 		'shader/nvvertparse.c',
 		'shader/program.c',
 		'shader/prog_cache.c',
-		'shader/prog_debug.c',
 		'shader/prog_execute.c',
 		'shader/prog_instruction.c',
 		'shader/prog_noise.c',