anv: Drop alpha_to_coverage from the NULL FS optimization

anv: Drop alpha_to_coverage from the NULL FS optimization

Starting with Ivy Bridge, we implement alpha-to-coverage by writting
gl_SampleMask with a pattern based on alpha.  This will show up in
wm_prog_data::uses_omask so we don't need to look at the key.

Fixes: 36ee2fd61c8f ("anv: Implement the basic form of VK_EXT_transform_feedback")
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16506>