Remove source_under_test to comply with new BUCK version

Change-Id: Ie3dd9fc9087c6cb0a0b5ba465c8910e91d982343

BUCK

diff --git a/BUCK b/BUCK
index 538d217..8865738 100644
--- a/BUCK
+++ b/BUCK
@@ -37,5 +37,4 @@
   resources = glob(['src/test/resources/**/']),
   labels = ['websession-flatfile'],
   deps = GERRIT_TESTS + DEPS,
-  source_under_test = [':websession-flatfile__plugin'],
 )