Remove never-used IMAP_XOAUTH2 option value before it gets more confusing
Before we knew the full wackiness splendor of OAUTH2, there was an attempt to model it as its own protocol. This never worked so as the 5.23 release was going to press the as-yet-unreleased mail protocol option value was disabled, but left in the database.
Now that (inbound) OAUTH2 is working using a different mechanism, remove this option value before it starts causing confusion.