Drop planetcore_set_serial_speed() which had no users since its
inception in commit fec60470 ("[POWERPC] bootwrapper: Add PlanetCore
firmware support") in 2007.
Signed-off-by: NArseny Solokha <asolokha@kb.kras.ru>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>