Remove unused variable from run-tox.sh

The recently landed change I1fc3b146e99e3484fa009b227ee5d0aeb6ed6084
set a variable stestr which is never used anywhere. This was a leftover
from an earlier revision of the patch. This commit just removes that
since it's not needed at all.

Change-Id: I7f7d03f79a7c020506acda5d3532f72efcc2813e

jenkins/scripts/run-tox.sh

@@ -32,7 +32,6 @@ function freeze_venv {
 function process_testr_artifacts {
     if [ -d ".stestr" ] ; then
         if [ -f ".stestr/0" ] ; then
-            stestr=1
             $bin_path/stestr last --subunit > ./testrepository.subunit
         fi
     else