remove Assertions interface from chai

- this interface is not properly typed and has implicit any errors.
  moroever, it types the chai-jquery plugin, not chai itself. so, I
  don't think this should be present with the main chai.d.ts. we can
  create a chai-jquery.d.ts if anyone needs it in the future. for now,
  I would like this file to work under 1.0.0.
- also, specify return type for "use" to avoid implicit any errors