integrity: powerpc: Do not select CA_MACHINE_KEYRING

No other platform needs CA_MACHINE_KEYRING, either.

This is policy that should be decided by the administrator, not Kconfig
dependencies.

Cc: stable@vger.kernel.org # v6.6+
Fixes: d7d91c47

 ("integrity: PowerVM machine keyring enablement")
Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>