systemctl: remove extra padding from status output

In 131601349 'systemctl: align all status fields to common column',
padding was calculated for 'ListenStream: ...', etc. Later on in
45a4f7233 'systemctl: tweak output of Listen: fields a bit' output
was changed to 'Listen: ... (stream)', but calculation didn't change.

Just remove the calculation, since now the result will be always 8,
and it it more important to have everything aligned to the widest
field ("Main-PID"), than to save a few columns, usually at most two
(e.g. "Listen").

Note: strlen is more natural, and is optimized to sizeof even
with -O0.