Remove default license from the Package type #12

Right now we default to the MIT license for packages that don't specify it:
https://github.com/purescript/registry/blob/e90a200ddcd2216258ec78145014f0fad5c5a6eb/v1/Registry.dhall#L32-L36

However, as noted in #4:

I don't think we should have a default for the license field; picking a license should be something we require the maintainer to actively do, I think.

..so we should remove it from the defaults and see if there's any issues with packages imported from Bower (this will require tweaking the curator here)