Revert 'unknown_sign_in_email' change, following upstream

This change, see upstream https://gitlab.com/gitlab-org/gitlab/-/merge_requests/46018,
didn't make it to releases, but we had it in our release branch.

The bad part is that it relies on a method that doesn't exist on this
version (`password_authentication_enabled_for_web?`), hence it
probably was breaking all login notifications.

--HG--
branch : heptapod-0-21
29 jobs for heptapod-0.21.1 in 21 minutes and 12 seconds (queued for 3 seconds)
latest
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tarball Upload
+++ b/HEPTAPOD_RAILS_SHA256	Wed Apr 28 23:00:04 2021 +0000
@@ -1,1 +1,1 @@
-2d42b710fe8292a84dbc5568f4cf90330099d816ebbeca7b9634584c4875394b
+9fbbfc88fb24dc9b2256a1dfe09105d9c54948492a15659cc8f92cf58d2682ff
Attempting to create tag heptapod-0.21.1-1. In case of failure,
especially because the tag already exists, please finish manually.
abort: tag 'heptapod-0.21.1-1' already exists (use -f to force)
Cleaning up file based variables
ERROR: Job failed: exit code 1