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

arch/powerpc/mm/book3s64/pkeys.c

@@ -307,16 +307,6 @@ void thread_pkey_regs_init(struct thread_struct *thread)
 	write_iamr(pkey_iamr_mask);
 }
 
-static inline bool pkey_allows_readwrite(int pkey)
-{
-	int pkey_shift = pkeyshift(pkey);
-
-	if (!is_pkey_enabled(pkey))
-		return true;
-
-	return !(read_amr() & ((AMR_RD_BIT|AMR_WR_BIT) << pkey_shift));
-}
-
 int __execute_only_pkey(struct mm_struct *mm)
 {
 	return mm->context.execute_only_pkey;