Remove webstore code for signin that is no longer used.

Now that the webstore does not require sign in for apps, the code to
support the signin as part of install flow is no longer required.

Some code is still needed to support paid apps.

BUG=388083

Review URL: https://codereview.chromium.org/482293003

Cr-Commit-Position: refs/heads/master@{#291875}
4 files changed