Remove confusing “You are already signed in.” flash message (#13547)

Remove confusing “You are already signed in.” flash message (#13547)

When attempting to access the log-in page while already logged in,
Devise's `require_no_authentication` kicks in and sets a flash
message “You are already signed in.”

In almost all cases, this also causes a redirect to /web, which
does not display or clear flash messages, thus leaving the message
to a potentially much later date, like for instance, accessing
/preferences several minutes after being redirected to /web.