core: don't log job status message in case job was effectively NOP (#3199)

core: don't log job status message in case job was effectively NOP (#3199)

We currently generate log message about unit being started even when
unit was started already and job didn't do anything. This is because job
was requested explicitly and hence became anchor job of the transaction
thus we could not eliminate it. That is fine but, let's not pollute
journal with useless log messages.

$ systemctl start systemd-resolved
$ systemctl start systemd-resolved
$ systemctl start systemd-resolved

Current state:
$ journalctl -u systemd-resolved | grep Started

May 05 15:31:42 rawhide systemd[1]: Started Network Name Resolution.
May 05 15:31:59 rawhide systemd[1]: Started Network Name Resolution.
May 05 15:32:01 rawhide systemd[1]: Started Network Name Resolution.

After patch applied:
$ journalctl -u systemd-resolved | grep Started

May 05 16:42:12 rawhide systemd[1]: Started Network Name Resolution.

Fixes #1723