Remove EXT_convolution.

Optional OpenGL 1.2 extension, that's not really supported anymore.
We are not doing anyone a favour by keeping it.

http://lists.freedesktop.org/archives/mesa-dev/2010-September/003098.html
Remove EXT_histogram.

Another part of ARB_imaging, we are dropping support for.
http://lists.freedesktop.org/archives/mesa-commit/2010-September/025253.html
Drop SGI_color_table and EXT_color_subtable optional 1.2 extensions.

Same as before, part of ARB_imaging subset that is deprecated and
removed from all major drivers.