util.stanza: Remove dead code
These cases are caught by `check_text(v, ..)` above. Those errors do not
contain the attribute however, which would have been nice.
For full information on our dependencies, version requirements, and
where to find them, see https://prosody.im/doc/depends
If you have luarocks available on your platform, install the following:
- luaexpat
- luasocket
- luafilesystem
- luasec