Remove support for the “!” separator to address IRC users

There’s currently two ways to address the user “NickName” on the server irc.example.com:

nickname!irc.example.com@biboumi.example.com irc.example.com@biboumi.example.com/NickName (or #anychannel%irc.example.com@biboumi.example.com)

We should remove the support for the first one because:

• it doesn’t work with nicknames containing characters that are not allowed in a JID nodepart (for example @ or /)
• it is really confusing in the fixed_irc_server mode (it’s nickname!@biboumi.example.com)
• the code would be simpler without it
• it’s simpler and less confusing for the user to have a single way to do one thing

(from redmine: created on 2015-05-29)