casetransform: optimize out capture groups

Created by: keegancsmith

Zoekt exposes OptimizeRegexp which is targets transformations which make the go regex engine faster for text search use case. Right now this is just transforming capture groups into non-capture groups. This is safe for casetransform since it doesn't expose subgroups in its API.

The only user of casetransform.Regexp is gitserver search, so this might translate into a noticeable difference for queries using groups.

Test Plan: CI, optimize regexp has already had its own testing.