docs/GL3: don't list swrast
docs/GL3: don't list swrast

Let's face it: This driver is unlikely to get more love.

Reviewed-by: Ilia Mirkin <bcbbb7aa705ec3a5fc5824c01a845186c3d62fca@alum.mit.edu>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler