Fix buck build, remove java_test.source_under_test

This parameter is also not required any more after buck upgrade
in Gerrit.

Change-Id: I32d9e6dcc49c3af0c32033824ed673f82270e9d7
(cherry picked from commit 2e89992f6b5bd9a5bdf691a8a51876dcfc565040)
diff --git a/BUCK b/BUCK
index ebabea3..a29218e 100644
--- a/BUCK
+++ b/BUCK
@@ -28,5 +28,4 @@
   srcs = glob(['src/test/java/**/*.java']),
   labels = ['delete-project'],
   deps = GERRIT_PLUGIN_API + GERRIT_TESTS + [':delete-project__plugin'],
-  source_under_test = [':delete-project__plugin'],
 )