selftests/powerpc: Remove powerpc special cases from stack expansion test

Now that the powerpc code behaves the same as other architectures we
can drop the special cases we had.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200724092528.1578671-5-mpe@ellerman.id.au