Remove unused JNITest class

JNITest class is no longer actively used. This patch
removes the class (java and jni) files.

JNI interfaces and calls are extensively tested in
the art unit tests (art/tests) and in cts (see
CtsJniTestCases).

Change-Id: I62f7c72deb5d206fa3f545ae39a9cb9011110d0a
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>