Delete apt update from travis config.

This is currently failing due to missing GPG pubkeys, and we don't think it's actually necessary.

PiperOrigin-RevId: 368673061

.travis.yml

@@ -28,7 +28,6 @@ env:
     # - PROJECT="unrestricted_advx"  # TODO(b/184862249): Fix and enable
     - PROJECT="unsupervised_adversarial_training"
 before_script:
-  - sudo apt-get update -qq
   - pip install --upgrade pip
   - pip install --upgrade virtualenv
   - pip install --upgrade wheel