Removing old code path for search_files_by_content

What does this MR do?

Removes the deprecated code paths to support a non-chunked response for SearchFilesByContent. The corresponding gitaly change is here: gitaly!1100 (merged)

Since the change to the SearchFilesByContent RPC has been deployed to prod, this is safe to remove now

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

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

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/56717

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer