Search backend: remove `textsearch` package

Created by: camdencheek

The internal/search/textsearch package only contains tests. The test covers both zoekt and searcher jobs, so I moved it into the jobutil package. The test itself is of questionable utility since it mostly reimplements our repo subset text search logic, but not in exactly the same way as our repo subset search logic works anymore. This test is probably a candidate for rewriting or deletion.

Stacked on https://github.com/sourcegraph/sourcegraph/pull/39538

Test plan

Just moving stuff around.