Drop support for older Barback versions.

This also allows us to limit our tests to the latest barback, which
simplifies the necessary infrastructure and speeds up the tests
dramatically.

Closes #1367

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1530353002 .

Author: nex3

.gitmodules

@@ -1,7 +1,4 @@
 {
-  "pre_test_hooks": {
-    "Update submodules": "git submodule update --init --force"
-  },
   "test_package": {
     "platforms": ["vm"]
   }