mesa: Remove EXT_texture_env_combine extension enable flag

mesa: Remove EXT_texture_env_combine 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.  The existing support is already partially
broken in Mesa (e.g., querying GL_TEXTURE_ENV_MODE in OpenGL ES 2.x).
This patch does not change the situation in any way.

It looks like the only hardware supported by Mesa that cannot do
ARB_texture_env_combine is pre-NV10 NVIDA chips.  It appears that
these chips cannot do the GL_SUBTRACT mode.  Based on looking at older
copies of nvOpenGLspecs.pdf found on the net, NVIDIA never supported
ARB_texture_env_combine on those chips either.

This extension was previously not supported on mach64, mga (G200),
r128, 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>