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