Remove unused "page" form variable from POST requests:
This is ancient code from back when page POST was handled like that: https://github.com/freenet/plugin-WoT-staging/blob/7080cd0315726adfb553ccd5b6d2159169b14ea5/ui/web/WebInterface.java#L88
We now have Toadlets which are accessed by URI, so we don't need the "page" variable anymore.