powerpc/pkeys: remove unused pkey_allows_readwrite

pkey_allows_readwrite() was first introduced in the commit 5586cf61
("powerpc: introduce execute-only pkey"), but the usage was removed
entirely in the commit a4fcc877 ("powerpc/pkeys: Preallocate
execute-only key").

Found by the "-Wunused-function" compiler warning flag.

Fixes: a4fcc877 ("powerpc/pkeys: Preallocate execute-only key")
Signed-off-by: Qian Cai <cai@lca.pw>
Acked-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1568733750-14580-1-git-send-email-cai@lca.pw