Delete NetworkManagementSocketTagger.

This code is no longer used:

- Clearing counters and deleting tag data was moved to
  NetworkStatsService in aosp/1962922 .
- Tagging and untagging sockets was moved to TrafficStats in
  aosp/1964912 . That CL deleted the .cpp file, but forgot to
  delete the .java file.

Test: m
Change-Id: Idc9acea09cf75295e22fba5904cce7f3acdd6342