st/dri: remove GALLIUM_MSAA and __GL_FSAA_MODE environment variables

Some users don't understand that these variables can break OpenGL.
The general is rule is that if an app supports MSAA, you mustn't use
GALLIUM_MSAA.

For example, if an app has an 8xMSAA FBO and GALLIUM_MSAA=4
is set, resolving the FBO to the back buffer will be rejected which will look
like this on all gallium drivers:

http://www.phoronix.com/scan.php?page=article&item=amd_radeonsi_msaa

The environment variables also have no effect on modern apps like TF2, but
there is still a performance hit due to wasted bandwidth and VRAM.

In a nutshell, it does more harm than good.

Cc: 10.2 10.3 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
(cherry picked from commit 8449121971ce1db03fea19665d314e523fdc10dd)