Bug 1793560 - Remove navigate-to CSP directive r=tschuster

It has never shipped after being implemented years ago,
and was removed from spec in September 2022:
https://github.com/w3c/webappsec-csp/pull/564

Now skipping navigate-to WPT tests. Filed issue upstream for their future removal:
https://github.com/w3c/webappsec-csp/issues/608
Consensus seems to agree to remove, will do in follow up bug once landed.

Also removed our own tests.

Added a hack in StartDocumentLoad as just removing the navigate-to check call
breaks some inhertiance, see comment for more info.

Differential Revision: https://phabricator.services.mozilla.com/D181630

netwerk/base/nsILoadInfo.idl

@@ -711,7 +711,6 @@ interface nsILoadInfo : nsISupports
 
   /**
    * True if the load was initiated by a form request.
-   * This is important to know to handle the CSP directive navigate-to.
    */
   [infallible] attribute boolean isFormSubmission;