chore(travis): remove BrowserStack credentials

Closes angular#5596

Author: vojtajina

.travis.yml

@@ -9,8 +9,6 @@ env:
   global:
     - SAUCE_USERNAME=angular-ci
     - SAUCE_ACCESS_KEY=9b988f434ff8-fbca-8aa4-4ae3-35442987
-    - BROWSER_STACK_USERNAME=VojtaJina
-    - BROWSER_STACK_ACCESS_KEY=HAfHZaypxAc3PEUrUU9a
     - LOGS_DIR=/tmp/angular-build/logs
     - BROWSER_PROVIDER_READY_FILE=/tmp/sauce-connect-ready