trackfilter: Don't require unused time values for each trackpoint

[jidanni]

$ ... -x track,sdistance=1k ...
trackfilter-init: Found track point at ... without time!

The program should only throw up its hands in this case if the user is requesting
time based filtering, but here he is only requesting distance based filtering.

Here it is asking the user to add timestamps that will not be used.

Besides, the track was made via -x transform,trk=wpt, which doesn't
necessarily add timestamps in the first place anyway.

Wait. There's the

faketime option

The parameter This option(!!) was added because some software products (e.g. garmin training center) require a time value for each trackpoint.

Therefore please don't make the same mistake.