These were intended to make it possible to access these settings
in the MT settings UI. However, apparently it's not possible to
have nil as a default, and MT throws warnings in this scenario.
Remove these settings until that's properly supported.
These are all based on automated detection (intercept the minetest
settings metatable methods) and lack any useful documentation, and
may be incomplete (e.g. missing keys that are only discovered at
runtime based on specific actions).
Eventually these will be filled out and used to document settings,
and make it easier to manage settings in the MT Settings UI.
