starter: Remove IPsec stack detection

starter: Remove IPsec stack detection

Checking specifically for /proc/net/pfkey is not ideal as af_key will
eventually be removed in Linux kernels.  Support for KLIPS is long gone.
The detection also wasn't used for anything anymore (failures were just
ignored since the ports to BSD-based systems).  And modprobing doesn't seem
to be necessary either (charon-systemd doesn't do that, for instance).