nixos/plasma5: do not set kimpanel as default IBus panel

kimpanel does not show installed IBus engines or allow switching input
methods. kimpanel does show configured keyboard layouts through kxkb, so I
believe there is some problem communicating with IBus. No error messages are
produced in the log and I have been unable to discover the cause. I have no
intention of continuing to work on kimpanel at this time, so it should be
disabled. The GTK+ 3-based panel provided by IBus is perfectly serviceable in
the interim.