Simplify twrpTar skip dir checks, DU code cleanup

* Remove unused parent variable from twruDU constructor

* Replace manual checks for /data/media with DU's check_skip_dirs()
  function

* Remove check_skip_dirs(A,B) function which duplicates the
  functionality of check_skip_dirs(A)

* Remove redundant trailing slash removal from
  check_absolute_skip_dirs(); this function is only accessed through
  check_skip_dirs()

Change-Id: Icfe8d71de36e33db6749fc5896dd3eafbcf7c9a4