arch/parisc: Removing undead ifdef CONFIG_PA20

The CONFIG_PA20 ifdef isn't necessary at this point, because it is
checked in an outer ifdef level already and has no effect here.

Signed-off-by: Christian Dietrich <qy03fugy@stud.informatik.uni-erlangen.de>
Signed-off-by: Kyle McMartin <kyle@redhat.com>