main: don't show help text anymore when we detect an unknown kernel cmdline value...

main: don't show help text anymore when we detect an unknown kernel cmdline value starting with "systemd."

As generators and other components started to maintain their own kernel
command line options this help text needed more and more exceptions and
wasn't complete anyway. Fixing that would leak more information about
specific generators into PID 1, which we should avoid.

Given that kernel cmdline handling traditionally doesn't generate errors
or show help texts, let's just remove the logic for it for systemd too.