core: don't update connection timestamps periodically every 300 seconds

We store the timestamp when a profile activated the last time to
"/var/lib/NetworkManager/timestamps". There was also a timer which
would update the timestamp of activated connections every 300 seconds.

That seems unnecessary, drop it.

For one, waking up every 5 minutes and rewriting a file to disk seems
undesirable, for example if /var is a device where unnecessary writes
should be minimized.

Note that we already update the timestamp when a device goes down,
and of course when it comes up. Updating the timestamp in between seems
unnecessary.

This reverts commit 60735029 ('core: update timestamp in active
system connections every 5 mins (bgo #583756)').

An alternative would be to only update the timestamp in memory (so that
it would appear updated on D-Bus), but delay writing the file until
something important happens. `nm_key_file_db_*()` already tracks whether
there are changes ("dirty") and whether it's necessary to write the
file. It would be possible to track two dirty flags: one that requires
immediate update, and one that only ensures we will re-write dirty files
eventually.

See-also: https://bugzilla.gnome.org/show_bug.cgi?id=583756

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