Drop 'isPromise'

We shouldn't do any additional validation beyound TS

src/components/Voyager.tsx

@@ -89,7 +89,7 @@ export default class Voyager extends React.Component<VoyagerProps> {
 
     let promise = this.props.introspection(getIntrospectionQuery());
 
-    if (!isPromise(promise)) {
+    if (!promise instanceof Promise) {
       throw new Error(
         'SchemaProvider did not return a Promise for introspection.',
       );
@@ -249,8 +249,3 @@ export default class Voyager extends React.Component<VoyagerProps> {
     return props.children || null;
   };
 }
-
-// Duck-type promise detection.
-function isPromise(value) {
-  return typeof value === 'object' && typeof value.then === 'function';
-}