Don't link java tests against junit by default

There is more variety of java tests than I expected, don't
default to including junit, that's just going to lead to
unexpected junit classes for module authors that don't know
to set junit: false.  All existing uses of java_test are
already not using junit or setting static_libs: ["junit"],
and all test in Make are already specifying junit.

Bug: 70770641
Test: m checkbuild
Change-Id: I4393b70d87dd2b6e3bb719fdb758915053bee0c7