Tcx: fix speed in summary information

When exporting files to Tcx, it's writing an ActivityExtension to the end
of the file with calculated AvgSpeed and MaxSpeed. Unfortunatly it's using
the km/h values - while Tcx is using m/sec. Schema is a bit unclear about
this, as the ActivityExtension uses "double" as type for both values, but
original Tcx v2 Schema only uses m/sec, GTC exports as m/sec and some
other software is expecting m/sec, aswell.
Tcx: suppress HeartRate tag completely

writing a Heartrate of 1 for unset values is just a workaround that
introduces bad data for programs not filtering invalid data.

Correct way is to suppress the HeartRate Element, completely.

Current Trainingscenter is still showing Heartrate graph for files with
partly missing heartrate in my tests.