KEYS: remove CONFIG_KEYS_COMPAT

KEYS_COMPAT now always takes the value of COMPAT && KEYS.  But the
security/keys/ directory is only compiled if KEYS is enabled, so in
practice KEYS_COMPAT is the same as COMPAT.  Therefore, remove the
unnecessary KEYS_COMPAT and just use COMPAT directly.

(Also remove an outdated comment from compat.c.)

Reviewed-by: James Morris <jamorris@linux.microsoft.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Tested-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>