In branch 5.2 we erase `dc` from `_datacenters` if there are
no more endpoints listed in `_dc_endpoints[dc]`.
This was lost unintentionally in f3d5df5448
and this commit restores that behavior, and fixes test_remove_endpoint.
Fixesscylladb/scylladb#14896
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Closes#14897