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/plugin.tsx

@@ -277,16 +277,8 @@ export class FlipperPlugin<
   client: PluginClient;
   realClient: Client;
 
-  /**
-   * @deprecated use .device instead
-   */
-  getDevice(): Promise<BaseDevice> {
-    return this.realClient.device;
-  }
-
   get device() {
-    // eslint-disable-next-line node/no-sync
-    return this.realClient.deviceSync;
+    return this.realClient.device;
   }
 
   _teardown() {