Arcyd: combine branch deletes when landing

Combine the two branch deletes during landing to reduce the exposure to
the Git service.

Re-order the paramaters to phlgit_push.delete() to allow for multiple
branch names.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh
poke, accept and see branches deleted in
$ ./testbed/arcyd/test_shell.sh