Remove invisible presence fields from c2s #state

Invisible presence isn't supported [anymore][1], so the corresponding `ejabberd_c2s` `#state` fields were unused.  Removing them simplifies the code a bit and saves a few bytes.

[1]: https://github.com/processone/ejabberd/commit/9deb294328bb3f9eb6bd2c0e7cd500732e9b5830