enb_registry: fix pattern match in handle_info 'DOWN' handler
PIDs maps pid() => enb_handle() (an integer), so maps:find/2 returns {ok, SomeHandle}. The old pattern {ok, Pid} tried to match an integer against the already-bound pid() variable, which never succeeds.
This handler is only called on abnormal termination (process crash), so the broken match caused the registry entry to never be cleaned up in that case. Fix by using a fresh {ok, Handle} binding and removing the now-redundant follow-up maps:get/2 call.
