messageTray: Always hide while the OSK is up

The message tray currently operates in three modes: in the overview,
normal, and while the on-screen keyboard is up. The last case is
particularly odd, and exclusively used for chat-notifications. As
users can still use the Chat application directly on touch-only
devices, the additional mode isn't really justified, so remove it.

https://bugzilla.gnome.org/show_bug.cgi?id=662687