Remove Westmint runtime

[gilescope]

We should not need to build a separate runtime. We should be able to throw statemint runtime with a westmint chain-spec without needing to build and maintain a separate runtime.

Only one catch: I think we need to fix #885 before we can do this as we don't want to make westmint use ed.

[gilescope]

There's risk in moving statemint back to SR. Not sure at the moment that the risks outweigh the rewards.