Some Client related cleanups

Summary:
Client up `client.device` (which had no code references anymore) / `client.deviceSync`. Cleaned up feature code for old SDKs (pre 2, which is 3 years old).

This makes decapitating Client a little simpler in the rest of the stack.

Reviewed By: passy

Differential Revision: D31235436

fbshipit-source-id: 919679c1830e2b9368d0787d7b363c090305edb8

desktop/app/src/test-utils/MockFlipper.tsx

@@ -192,12 +192,6 @@ export default class MockFlipper {
       new Set(supportedPlugins),
       device,
     );
-    // yikes
-    client.device = {
-      then() {
-        return device;
-      },
-    } as any;
     client.rawCall = async (
       method: string,
       _fromPlugin: boolean,