Revert "Add imaplib.IMAP4.abort to exception classes for retry_crispin."

imaplib.IMAP4.abort is actually a subclass of imaplib.IMAP4.error, so adding it
to CONN_DISCARD_EXC_CLASSES wasn't doing anything.

This reverts commit 230c0f7ddc3effdfe8479611609105d71e5f1617.