Arcyd: remove tests for refs/config/arcyd

We're not using the refs/config arcyd feature at the moment and testing
it is going to get in the way of the new CLI, which doesn't support it
yet.

Test Plan:
$ ./testbed/arcyd/acceptance_tests.sh
Arcyd: BREAK, remove 'arcyd process-repos'

Remove the 'arcyd process-repos' command now that 'arcyd start' has as
much feature parity as we want.  Replace all usage of 'process-repos' in
test scripts with the new CLI commands.

Maintaining both will be more expensive going forward, so we'll remove
it now.

Existing instances will need to be re-created using the new CLI commands
built for that purpose: init, add-phabricator, add-repo, start, stop

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