mesa: Remove EXT_blend_subtract extension enable flag

mesa: Remove EXT_blend_subtract extension enable flag

All drivers remaining in Mesa support this extension.  This extension
is either required or optional features in desktop OpenGL, OpenGL ES
1.x, and OpenGL ES 2.x.

This extension was previously not supported on i810, mach64, mga,
savage, sis, and tdfx (Voodoo Banshee and Voodoo3).

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>