Remove use of junit4-target when used with junit-targetdex

Change to external/junit means that junit-targetdex now includes
everything that is in junit4-target so it is no longer necessary
to use them both.

Bug: 30188076
Test: make checkbuild and ran cts-tradefed -m CtsLibcoreOjTestCases
Change-Id: I6f7107cf111b0b2b437d38e48ee590846e991ecf