Adjust to latest buck version

Remove "source_under_test" parameter.

Change-Id: I32804f39168e8790735086d5cc4feccd2eb848af
diff --git a/BUCK b/BUCK
index db7add4..9d8c705 100644
--- a/BUCK
+++ b/BUCK
@@ -41,7 +41,6 @@
   name = 'uploadvalidator_tests',
   srcs = glob(['src/test/java/**/*.java']),
   labels = ['uploadvalidator'],
-  source_under_test = [':uploadvalidator__plugin'],
   deps = GERRIT_PLUGIN_API + GERRIT_TESTS + [
     ':commons-io',
     ':uploadvalidator__plugin',