Refactor: libcrmservice: remove dead code in action_exec_helper()

Previously, action_exec_helper() had a fall-through case for when the resource
standard was systemd or upstart, but was not supported. However,
services__create_resource_action() will return an error in that case, so
action_exec_helper() should never see it.

Also, use pcmk__str_eq() instead of strcasecmp().