Fixed gz file-already-exist issue on log-collection for periodic-voltha-sanity-test-multi-runs job

Change-Id: I1e285bacf80754a96c754f4799ae45d763c39f31
diff --git a/jjb/pipeline/voltha/master/bbsim-tests.groovy b/jjb/pipeline/voltha/master/bbsim-tests.groovy
index c33d24a..b01d88a 100755
--- a/jjb/pipeline/voltha/master/bbsim-tests.groovy
+++ b/jjb/pipeline/voltha/master/bbsim-tests.groovy
@@ -131,6 +131,7 @@
         done
       fi
       cd $WORKSPACE/${testTarget}-components/
+      rm onos-voltha-combined.log.gz || true
       gzip -k onos-voltha-combined.log
       rm onos-voltha-combined.log
     """