Remove pubsub unregistering when module is disabled for tenant

According to investigation in MODPUBSUB-147, when module is restarted with the same version it unregisters itself from pubsub during DELETE _tenant and doesn't get registered again because Okapi doesn't make a call to POST _tenant endpoint in this case. This leads to mod-circulation being unable to publish events. Pubsub developers say that unregistering is unnecessary and some other modules don't do it.

The goal of this spike is to determine if removing the unregistering logic helps to fix this issue and, if it does, fix it in scope of this ticket too.