mod_mam_archive: remove invalid disco#info feature The feature which was previously added to disco#info was in fact specified to be used for stream features only (see XEP-0136 section 11). Emitting it in disco#info is weird at best and breaks stuff at worst.

This module discards typing notifications sent to a bare host JID,
preventing error replies to be sent. These errors are harmless but can
be annoying sometimes if your client shows them prominently.