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/flipper-plugin/src/test-utils/test-utils.tsx

@@ -204,7 +204,7 @@ export function startPlugin<Module extends FlipperPluginModule<any>>(
       device_id: testDevice.serial,
       os: testDevice.serial,
     },
-    deviceSync: testDevice,
+    device: testDevice,
     isBackgroundPlugin(_pluginId: string) {
       return !!options?.isBackgroundPlugin;
     },