The FetchInternalRemote RPC had been used internally to replicate
repositories across different Gitaly nodes. At some point in time, this
was converted to do a direct fetch though without an additional RPC call
because as it turned out, doing inter-Gitaly mutating RPC calls is
problematic in combination with transactions given that the remote side
would now try to cast votes against another Gitaly. Nowadays, there are
no callers left which use this RPC.

Remove the deprecated `FetchInternalRemote()` call. The backing logic
which is still internally called remains though for use in other parts
of Gitaly. We may eventually want to find a better place for it to live.

Changelog: removed