Revert "Add a db-relation-broken hook"

Turns out this was not a good idea and didn't work. Need to figure out something else how to deal with the situation where the diaspora service is destroyed but the postgresql service is not - and then diaspora is redeployed.

This reverts commit 3fd532914936d7ded2e5f4cae40863ccc3076be1.
Clarify readme relating to problem with destroying diaspora service