Really fix login form regression

Remove 'form' from allowed attributes list of input field. We needed
it only for a moment, now we don't. It's also not supported by IE/Edge
which means there's no way we would need it. Maybe some day...

program/lib/Roundcube/html.php

@@ -396,7 +396,7 @@ class html_inputfield extends html
         'type','name','value','size','tabindex','autocapitalize','required',
         'autocomplete','checked','onchange','onclick','disabled','readonly',
         'spellcheck','results','maxlength','src','multiple','accept',
-        'placeholder','autofocus','pattern','form',
+        'placeholder','autofocus','pattern',
     );
 
     /**