docs/GL3: don't list r300
docs/GL3: don't list r300

r300g already supports everything it can. There's no point in listing
the driver here.

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

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler