Fix buck build, remove java_test.source_under_test

This parameter is not required any more after latest buck upgrade.

Change-Id: I17eba90f486230ea4f6118d9fd99addd0f8fb344

BUCK

diff --git a/BUCK b/BUCK
index 7a7232b..e336c73 100644
--- a/BUCK
+++ b/BUCK
@@ -44,7 +44,6 @@
   name = 'sync-index_tests',
   srcs = glob(['src/test/java/**/*.java']),
   labels = ['sync-index'],
-  source_under_test = [':sync-index__plugin'],
   deps = TEST_DEPS,
 )