selftests/mm: remove empty pkey helper definition

Some of the functions declared in pkey-helpers.h are actually defined in
protections_keys.c, meaning they can only be called from
protections_keys.c.  This is less than ideal, but it is hard to avoid as
these helpers are themselves called from inline functions in
pkey-<arch>.h.  Let's at least add a comment clarifying that.  We can also
remove the empty definition in pkey_sighandler_tests.c:
expected_pkey_fault() is not meant to be called from there.

Link: https://lkml.kernel.org/r/20241209095019.1732120-10-kevin.brodsky@arm.com


Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Keith Lucas <keith.lucas@oracle.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>