Fix buck build, remove java_test.source_under_test

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

Change-Id: Ideaa1fed62c20328e6383481cdae86527d960344

BUCK

diff --git a/BUCK b/BUCK
index 367d58e..d6f1201 100644
--- a/BUCK
+++ b/BUCK
@@ -48,7 +48,6 @@
   name = 'sync-events_tests',
   srcs = glob(['src/test/java/**/*.java']),
   labels = ['sync-events'],
-  source_under_test = [':sync-events__plugin'],
   deps = TEST_DEPS,
 )