commit: Remove deprecated and unused CommitsBetween RPC
The CommitsBetween RPC has been deprecated in favor of ListCommits, which is a more flexible alternative. Rails has been converted to ListCommits in f1f39db10ee (Use the ListCommits RPC for Gitlab::Git::Commit.between, 2021-08-05) already, and with 96717bb (gitaly: Convert from CommitsBetween to ListCommits, 2021-11-12) the last existing callsite was removed from the Elasticsearch indexer.
Remove the RPC call completely.
Changelog: removed