Eliminate the LOGIN authentication option from SMTP

The LOGIN option is no longer offered to users as a choice.  This does
*not* eliminate the SASL LOGIN authentication mechanism.  Any pre-existing
LOGIN setting or any imported LOGIN setting will still be recognized.  In
all cases, a user setting of either "Normal password" or "LOGIN" will
result in the SASL PLAIN mechanism being tried first if available,
otherwise SASL LOGIN will be tried if available.

This mirrors similar behavior that exists for IMAP.