Remove dx's copy of JUnit.

Remove dx's copy of JUnit.

Instead, use JUnit the same way other code uses JUnit: by depending
on the copy in external. There is no longer a top-level option in
dx to run JUnit tests on itself.

There are two failing tests in dx/junit-tests. Both of these are in
the recently created DexGeneratorTest; I intend to fix these in another
change.

Bug: http://b/2286423
Change-Id: I3a173a0302c7ccc81be3b5a2a4d766000d1c242b