Revert "[iOS] CPE: Save newly generated credentials"

This reverts commit a55462e7.

Reason for revert: Failing on the bots, see comments.

Original change's description:
> [iOS] CPE: Save newly generated credentials
>
> The password itself is stored into the keychain, and the Credential
> object is saved to NSUserDefaults, so it can be read the next time
> Chrome starts.
>
> After saving finishes, the extension also passes the credentials back
> to the website, closing itself.
>
> Bug: 1224974
> Change-Id: I622954e9b734f07400798de84f6f6ed4d6d0f3da
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3058648


> Commit-Queue: Robbie Gibson <rkgibson@google.com>
> Reviewed-by: Javier Flores <javierrobles@chromium.org>
> Reviewed-by: Mark Cogan <marq@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#907168}

Bug: 1224974
Change-Id: I6344d176c30b42ffd92ba20a7cb41e0c28f6d8fe
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3065598


Auto-Submit: Justin Cohen <justincohen@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Javier Flores <javierrobles@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Javier Flores <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#907387}