Removed obsolete feature "Transfer TB-Tracking Number to Log-Field"

The above feature seams to be implemented by the GC website now. The tracking number is appended to the URL of the log page and there, it is inserted in the log field. Everything is done by the GC webserver.

Hence, the code block from line 4910 to 4922 dealing with the variable last_tracking_nr can be removed from GCLH!