powerpc/pkeys: Remove unused code

This removes arch_supports_pkeys(), arch_usable_pkeys() and
thread_pkey_regs_*() which are remnants from the following:

commit 06bb53b3 ("powerpc: store and restore the pkey state across context switches")
commit 2cd4bd19 ("powerpc/pkeys: Fix handling of pkey state across fork()")
commit cf43d3b2 ("powerpc: Enable pkey subsystem")

arch_supports_pkeys() and arch_usable_pkeys() were unused
since their introduction while thread_pkey_regs_*() became
unused after the introduction of the following:

commit d5fa30e6 ("powerpc/book3s64/pkeys: Reset userspace AMR correctly on exec")
commit 48a8ab4e ("powerpc/book3s64/pkeys: Don't update SPRN_AMR when in kernel mode")
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Reviewed-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210202150050.75335-1-sandipan@linux.ibm.com

arch/powerpc/include/asm/mmu_context.h

@@ -282,9 +282,6 @@ static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
 }
 
 #define pkey_mm_init(mm)
-#define thread_pkey_regs_save(thread)
-#define thread_pkey_regs_restore(new_thread, old_thread)
-#define thread_pkey_regs_init(thread)
 #define arch_dup_pkeys(oldmm, mm)
 
 static inline u64 pte_to_hpte_pkey_bits(u64 pteflags, unsigned long flags)

arch/powerpc/include/asm/pkeys.h

@@ -169,10 +169,4 @@ static inline bool arch_pkeys_enabled(void)
 }
 
 extern void pkey_mm_init(struct mm_struct *mm);
-extern bool arch_supports_pkeys(int cap);
-extern unsigned int arch_usable_pkeys(void);
-extern void thread_pkey_regs_save(struct thread_struct *thread);
-extern void thread_pkey_regs_restore(struct thread_struct *new_thread,
-				     struct thread_struct *old_thread);
-extern void thread_pkey_regs_init(struct thread_struct *thread);
 #endif /*_ASM_POWERPC_KEYS_H */