userMenu: Remove 'Switch Session' item

GDM does not allow concurrent logins of a single user, so making
'Switch Session' a user switch operation does not work - in order
to choose a different session, users have to log out.
Rather than making 'Switch Session' an alias of 'Log out' (which
is available anyway when multiple sessions are defined), remove
the item altogether - 'Switch Session' suggests an operation that
does not loose state, and we currently favor 'Switch Session' over
'Switch User', so on systems that have both multiple users and
multiple sessions, the latter would become unavailable.

https://bugzilla.gnome.org/show_bug.cgi?id=685062