pan/midgard: Remove unused variable

pan/midgard: Remove unused variable

../src/panfrost/midgard/mir.c: In function ‘mir_bytemask_of_read_components_index’:
../src/panfrost/midgard/mir.c:471:18: warning: unused variable ‘mask’ [-Wunused-variable]
  471 |         uint16_t mask = 0;

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3684>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3684>