chore(presubmit): Do not add a remote that is already added

Closes #1214

Author: jbdeboer

scripts/travis/presubmit.sh

@@ -24,7 +24,6 @@ if [ "$TRAVIS_REPO_SLUG" = "angular/angular.dart" ]; then
     git config user.name "[email protected]"
 
     echo "Pushing HEAD to master..."
-    git remote add upstream https://github.com/angular/angular.dart.git
     git stash
     git fetch upstream master
     git rebase upstream/master