mesa: Remove ARB_multitexture extension enable flag

mesa: Remove ARB_multitexture 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_CLIENT_ACTIVE_TEXTURE in OpenGL ES
2.x).  This patch does not change the situation in any way.

This extension was previously not supported on i810, mga (G200), or
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>