Clean up hwvtepsouthbound

General code clean-up:
* use optimal operations on maps: iterate over values if only values
  are needed; avoid double gets; use entry sets if both keys and
  values are needed;
* use the single-parameter variant of firstKeyOf();
* drop unused methods, fields and imports;
* limit variable scope;
* use Java 7 <> operator;
* since TyperUtils.extractRows...() never returns null, remove all
  such null-checks and rely on the for loops to check for emptiness.

Change-Id: Id9e4643a46a96d819146b04a146c2ac00c88509d
Signed-off-by: Stephen Kitt <skitt@redhat.com>