Revert "Temporary change to pull master branch for voltha-system-tests for voltha-2.7 jobs"

This reverts commit 53ee148688a643f479d934260276f9654bbf2d84.

Reason for revert: voltha-2.7 branch created for voltha-system-tests 

Change-Id: I841403d131049187f0a3ad1c953fc0c5bd775c06
diff --git a/jjb/pipeline/voltha-go-tests.groovy b/jjb/pipeline/voltha-go-tests.groovy
index dcea4c4..c8a80fd 100644
--- a/jjb/pipeline/voltha-go-tests.groovy
+++ b/jjb/pipeline/voltha-go-tests.groovy
@@ -77,7 +77,7 @@
             url: "https://gerrit.opencord.org/voltha-system-tests",
             // refspec: "${volthaSystemTestsChange}"
           ]],
-          branches: [[ name: "master", ]],
+          branches: [[ name: "${branch}", ]],
           extensions: [
             [$class: 'WipeWorkspace'],
             [$class: 'RelativeTargetDirectory', relativeTargetDir: "voltha-system-tests"],