ssh.service: avoid indirection via /bin/sh + do not run test mode in ExecStartPre

ssh.service: avoid indirection via /bin/sh + do not run test mode in ExecStartPre

There's no point in running ssh-keygen under /bin/sh, so let's drop this
unnecessary indirection.

Furthermore there shouldn't be any need to run `sshd -t` before starting
the ssh daemon on the live system, as we definitely would like to get
sshd started if it was requested to so, even if there might be a
(possibly intermittent) problem with it. The test run makes sense for
reloading an already running ssh daemon, and we keep this in place.

Thanks to András Korn for spotting this