Use gproc for subscribers inside the elarm registry

API change: No more subscription/unsubscription by proxy.

I prefer using plain gproc rather than gproc_ps as gproc_ps wraps the
message to publish in {?ETag, Event, Msg} - and this would change the
existing {elarm_started | elarm_down, Name} messages sent by
elarm_registry.
Ref: https://github.com/uwiger/gproc/blob/e5500cd5fb950813e60827d337767c0b4237aa74/src/gproc_ps.erl#L154