Remove `type` from metadata spec about mbtiles-spec

[robpvn]

There's a saying that goes " required is forever", and I think that removing something that is currently required is not a good idea once the spec is out in the wild. After all, there might be applications reading/writing this that you don't know about.

[tmcw]

@robpvn in protobuf, yes. But if we can find a relatively large segment of clients that aren't implementing this part of the spec, it makes sense not to have unnecessary historical baggage.

[incanus]

Uh, version it?

[robpvn]

Fair enough. Would it be cut out completely from the spec or moved to suggested? Because I can still see the rationale for having information about overlay or baselayer in an application that opens multiple MBTiles datasources. Obviously, if it is cut out from the spec it would still be possible for people to use it as an option but it would be less, shall we say, discoverable for implementors.