SqlPlaylist: don't observe track and re-save whole playlist on track change

No other playlist does that and this causes problems when someone has
really big playlists and slow connection to external MySQL db.

Not updating track metadata should not hurt, these are immediately
replaced by real ones by MetaProxy::Track. Moreover for uidUrl updates
there is SqlTrack::updatePlaylistsToDb() (I don't however know whether
it works as expected). Acked by Bart on the bug.

BUG: 301700
FIXED-IN: 2.6