Remove xml() Response method since it's not part of the spec or polyfills

see whatwg/fetch#16

Author: darknoon

lib/index.js

@@ -44,10 +44,6 @@ function response (httpResponse, data) {
         }
       })
     },
-    xml () {
-      // TODO https://developer.apple.com/reference/foundation/nsxmlparser
-      return Promise.reject('not implemented yet')
-    },
     blob () {
       return Promise.resolve(data)
     },