mesa: remove unnecessary checks in _mesa_readpixels_needs_slow_path

mesa: remove unnecessary checks in _mesa_readpixels_needs_slow_path

readpixels_can_use_memcpy will later call _mesa_format_matches_format_and_type
which does much tighter checks than these to decide if we can use
memcpy for readpixels.

Also, the checks do not seem to be extensive enough anyway, since we are
checking for signed/unsigned conversion only when the framebuffer has integers,
but the same checks could be done for other types anyway, since as long as
there is a signed/unsigned conversion we can't memcpy.

No regressions observed on i965/llvmpipe.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>