Remove FlowFixMe's from TypeScript

Summary: This diff was a joy to draft.

Reviewed By: jknoxville

Differential Revision: D17344676

fbshipit-source-id: 0694d0393fa243edef61408ecfee92912fbbfa32

src/reducers/plugins.tsx

@@ -63,7 +63,6 @@ export default function reducer(
         return;
       }
 
-      // $FlowFixMe Flow doesn't know prototype
       if (p.prototype instanceof FlipperDevicePlugin) {
         // @ts-ignore doesn't know p must be typeof FlipperDevicePlugin here
         devicePlugins.set(p.id, p);