Remove an obsolete ES5-ism

Remove ability to construct `RPC` class without calling `new`. This
confused TypeScript's inference of the types of various fields.

src/shared/frame-rpc.js

@@ -37,7 +37,6 @@ const VERSION = '1.0.0';
  *  @param {(Object<string, ((any) => any)>) | ((any) => any)} methods
  */
 export default function RPC(src, dst, origin, methods) {
-  if (!(this instanceof RPC)) return new RPC(src, dst, origin, methods);
   const self = this;
   this.src = src;
   this.dst = dst;