Fix errors reported by phpcs in includes/HTMLForm.php

I set property "$typeMappings" to private (no clients outside this class
in our codebase).

I removed property $jsAdded, it is unused in this class and throughout
the codebase and seems a leftover from 2008.
Special:Preferences still works with this change.

Change-Id: Id5668141835e22e3e1b043109e50d73ccd6b1229
Merge "Fix errors reported by phpcs in includes/HTMLForm.php"