[4.4] Upgrade "Login"/"Email address" string to use upstream code

DO NOT COMMIT until Bugzilla has been upgraded to 4.4.

Kill our custom hack to display "Email address" instead of
the confusing "login". This is a backport of 4.5 code in
http://bzr.mozilla.org/bugzilla/trunk/revision/8686

Change-Id: Ida6429cc3d7c38265a200573a5aaebf093464417