chore: Remove unused `Map` support from `next/dynamic` transform (#82487)
Noticed while skimming over this code.
It looks like `react-loadable` had a separate `Loadable.Map` API (https://github.com/jamiebuilds/react-loadable/tree/master?tab=readme-ov-file#loading-multiple-resources), but `next/dynamic` doesn't, so we shouldn't check for this.
The SWC version of this transform doesn't appear to have copied this code, and doesn't have the `Map` logic.