Deprecate non-chunking code path for SearchFilesByContent RPC

Once !1015 (merged) and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24111 are fully deployed, we can remove the non-chunking code path in the SearchFilesByContent RPC.

There are comments added to aid in this process

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information