Drop planetcore_set_serial_speed() which had no users since its
inception in commit fec6047047fd ("[POWERPC] bootwrapper: Add PlanetCore
firmware support") in 2007.
Signed-off-by: Arseny Solokha <asolokha@kb.kras.ru> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>