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 blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```