x86/pkeys: Remove __arch_set_user_pkey_access() declaration

In the x86 code __arch_set_user_pkey_access() is not used and is not
defined.

Remove the dead declaration.

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lkml.kernel.org/r/20220331180655.2946086-1-ira.weiny@intel.com