Revert 'unknown_sign_in_email' change, following upstream

This change, see upstream,
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.

branch : heptapod-0-21
29 jobs for heptapod-0.21.1 in 21 minutes and 12 seconds (queued for 3 seconds)
Name Stage Failure
tarball Upload
+++ b/HEPTAPOD_RAILS_SHA256	Wed Apr 28 23:00:04 2021 +0000
@@ -1,1 +1,1 @@
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