Remove non-chunked code path for SearchFilesByContent

Since the SearchFilesByContent change has been deployed to prod, we can now remove the legacy code path since we no longer need to support a "non-chunked" response.

closes #1464 (closed)

corresponding gilab-ce change: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25617

The last gitlab version that used non-chunked requests was 11.7.

In 11.8, we deployed 2cad0169 and https://gitlab.com/gitlab-org/gitlab-ce/commit/04b9de85a8355a9e9f4827fcd608c3ea2bcd7df0