Revert "Add hook to regain nickname in ERC"

Revert "Add hook to regain nickname in ERC"

This reverts commit 8c0c98268440b27a77faf30738dfd72c909bb33f.  The
functionality it introduced is likely being migrated to a
"regain"-specific local module to live in the library erc-services.el
alongside the existing `services' module.  Its scope will be expanded
to address common "regain" cases requiring NickServ.  This commit is
being reverted to prevent any confusion that might arise from users
encountering the old interface while the new one is being hashed out
on the tracker.