Code cleanup wrt checking if site authentication skipped option in SecurityValve

With HSTTWO-2229, the following was added in SecurityValve#checkAccess()
method:

if (requestContext.isCmsRequest())

{ if (requestContext.getResolvedMount().getMount().getVirtualHost().getVirtualHosts().isChannelMngrSiteAuthenticationSkipped()) { log.debug("Overriding authentication requirement because cms request"); return; }

}

But, basically #checkAccess() method is responsible for authorization (by
throwing security exception when disallowed), not authentication.
That's how it has been implemented.

HSTTWO-2229 seems about skipping authentication when
the request is from cms and the property is set.

So, the skipping code should be located outside #checkAccess() (Probably rather in the beginning of #invoke()).