r300: remove a set but not used variable

The use of phase_refmask was removed 12 years ago
in b7cf887ca74561469c144f1d12227e1bcf277e7e

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14252>
1 file changed