Remove discovery of generic pubsub support

We use PubSub only on our account jid, this use case is what PEP
was made for. If PEP is discovered we know that certain PubSub
features are supported, see: https://xmpp.org/extensions/xep-0163.html#defaults

The current check for <feature var='http://jabber.org/protocol/pubsub'/>
is pretty useless, as it just tells us that there is a PubSub implementation
but not much about the features. Only `publish` and `subscribe` are MUST
in XEP-0060 which is not enough for our needs.

If there is ever need to discover a generic PubSub implementation
that is not PEP we should check for all the PubSub features we need
instead of only for <feature var='http://jabber.org/protocol/pubsub'/>