powerpc/configs: Drop CONFIG_SERIAL_TXX9_* from cell/ppc64

In commit bf4981a0 ("powerpc: Remove the celleb support") we
dropped the celleb support, which made these symbols unselectable
because we no longer select HAS_TX99_SERIAL. So drop them from the
defconfigs.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>