mdn: clean up unused hacks

[myfreeer]

As yari goes more stable, some hacks for the old kuma-based mdn websites is no longer needed.
This is a proposal to check and remove all kuma-related code and hardcoded redirect code of removed contents.
The release 0.2.1 should be considered as last stable release with contents form kuma, and a possibly breaking release should be made before this is done.

[myfreeer]

This could make it incompatible with existing kuma-based servers (if any), should it be marked as breaking if implemented?