Replace AuthXoauth2 with Net::IMAP::SASL's version

The XOAUTH2 authenticator that comes from Net::IMAP::SASL does support
keyword attributes.  But XOAUTH2 is very simple, so this doesn't really
add much.

This is mostly to stay consistent with other SASL mechanisms and to
remove duplication:  We shouldn't have unique SASL mechanism
implementations for each individual protocol client.

Author: nevans