Don't show warning header for non-standard workflows on login-continue

Don't show warning header for non-standard workflows on login-continue

When a user needs/wants to login again (reauthentication or user account switch),
the Login special page shows a header to explain what happened ("already logged
in" or "need to re-authenticate"). For now, the form also displays this header
for a continued login workflow (e.g. when the user needs to enter secondary auth-
entication data, like a one time password). This is misleading, as the user already
knows what and why he has to login (again). Other error/warning messages are maybe
not as visible as they should be.

This change omits the header message for the login-continue auth action.

Bug: T146317
Change-Id: I2115bbe7965e8d0326f71872a4d33767dd0f956b