powerpc/qe: drop unused ucc_slow_poll_transmitter_now

Drop ucc_slow_poll_transmitter_now() which has no users since its
inception in 2007 in commit 98658538 ("[POWERPC] Add QUICC
Engine (QE) infrastructure").

Signed-off-by: Arseny Solokha <asolokha@kb.kras.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>