mesa: Remove EXT_texture_env_add extension enable flag

mesa: Remove EXT_texture_env_add 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.

This extension was previously not supported on mach64, mga (G200),
savage (Savage3D and other pre-Savage4), sis, and tdfx (Voodoo
Banshee).

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