i965: Remove GL_ARB_color_buffer_float from GL core contexts.

i965: Remove GL_ARB_color_buffer_float from GL core contexts.

Of the 3 controls in the extension, one was kept in GL core and the other
two were explicitly deprecated and the reasonable default behavior was
encoded in the spec.  By not exposing the extension, we avoid shader
recompiles when switching between float and unorm color buffers.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>