[signin] Delete pre-dice password signin promo

This code was supposed to be dead, but actually it was still called in
wrong situations:
- on ChromeOS, I could trigger it when launching without the user
  manager.
- when disabling the "allow browser signin" setting.

In both cases, clicking on the promo triggers a DCHECK.

Fixed: 1029315
Change-Id: If213b299a847a95b292f0c1d3c44b31d12cf6037
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1942657
Commit-Queue: David Roger <droger@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721969}
9 files changed