strongswan: swanctl.init: remove invalid privkeys option

strongswan: swanctl.init: remove invalid privkeys option

The 'swanctl.conf' has no 'privkeys' option in
'connections.<conn>.local<suffix>'. The only local-round keys documented
for authentication are 'certs', 'cert<suffix>' and 'pubkeys'. Strongswan
does not let a connection reference a private key by name at all - swanctl
auto-selects the matching private key from '/etc/swanctl/private'
(or rsa/ecdsa/pkcs8) by comparing it against the certificate configured
via 'certs' when credentials are loaded.

Writing 'privkeys = $local_key' into the generated swanctl.conf is
therefore a no-op at best: the option is unknown to the parser and
gets silently dropped, so it never had any effect on which key was
used.

Drop the bogus assignment. The existing local_key validation making
sure the referenced file exists under '/etc/swanctl/private'. Since that's
still useful to catch misconfiguration early, and add a comment explaining
why nothing is written to swanctl.conf for it.

Signed-off-by: Florian Eckert <redacted>