Bug 29956: Prevent login form to be serialized into cookie

Bug 29956: Prevent login form to be serialized into cookie

To recrate:
Logout
Go to /cgi-bin/koha/opac-search.pl
Click "Log in to your account"
Fill in the login form
Submit
Check the 'form_serialized' cookie's value

=> Without this patch it contain login/password
=> With this patch applied the cookie is not created

Confirm that the "Return to the last advanced search" feature still
works as expected.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>