[Passwords] Remove more insecure credentials-related code from PasswordStore

Bug: 1234334
Change-Id: Iac067a0d194ca76aac81eed7868a46b8e4d5dfb7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3081642


Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Ioana Pandele <ioanap@chromium.org>
Cr-Commit-Position: refs/heads/master@{#910214}