powerpc: Remove pte_ERROR()

pte_ERROR() is used neither in powerpc code nor in common mm code.

Remove it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/bec9eb973ecc1cba091e5c9201d877a7797f3242.1695659959.git.christophe.leroy@csgroup.eu