gallium/util: remove unused u_index_modify helpers

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
(cherry picked from commit 42297c862f60ccecf794297ee97410f007bd75df)
[Emil Velikov: Nominate as a requirement for next commit]
Nominated-by: Emil Velikov <emil.velikov@collabora.com>
2 files changed