In commit ebab4f48 (gitaly_client: Reimplement `#between` via
`#list_commits`, 2021-11-11), we have reimplemented `#between` via
`#list_commits` such that the deprecated CommitsBetween RPC isn't used
in GitLab Rails anymore. What was missed though is that `#between` isn't
being used at all anymore, and all previous callsites had already been
converted to use `#list_commits` in the first place. The calls which
were still visible in Grafana thus weren't caused by Rails, but they
were instead caused by the Elasticsearch indexer.

Remove `#between` altogether given that it's not in use. The feature
flag which has been introduced to convert this wrapper to instead use
`#list_commits` is removed, too.

Changelog: other