Arcyd: disable refs/config/arcyd monitoring

We're not using the repo options feature and it's quite expensive at
scale to do the 'git show-ref' query for every repo each cycle.

Later we can cache the results of those queries, disable the feature for
now to get cycle time down.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh
$ ./testbed/arcyd/loop_test.sh