powerpc: Remove find_current_mm_pte()

Last usage of find_current_mm_pte() was removed by
commit 15759cb0 ("powerpc/perf/callchain: Use
__get_user_pages_fast in read_user_stack_slow")

Remove it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ec79f462a3bfa8365b7df505e574d5d85246bc68.1646818177.git.christophe.leroy@csgroup.eu