Fix lint issues in CatalystInstanceImpl.java (facebook#55372)

Summary:

Fixed two lint warnings in CatalystInstanceImpl.java:
- NotInvokedPrivateMethod: Removed unused native method `unregisterFromInspector()`
- DeadVariable: Removed unused variable `wasIdle` while keeping the side effect of `mPendingJSCalls.getAndSet(0)`

changelog: [internal] internal

Reviewed By: javache

Differential Revision: D91795470

Author: mdvacca

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/bridge/CatalystInstanceImpl.java

@@ -344,7 +344,7 @@ public void destroy() {
           if (mFabricUIManager != null) {
             mFabricUIManager.invalidate();
           }
-          boolean wasIdle = (mPendingJSCalls.getAndSet(0) == 0);
+          mPendingJSCalls.getAndSet(0);
 
           getReactQueueConfiguration()
               .getJSQueueThread()