Remove unnecessary (and incorrect) code for compatibility with Paper …

…in the Fabric version of GlobalResponderHandler (facebook#26290)

## Summary

I'm working on a refactor of the definition of `Instance` in Fabric and
I came across this code that seemed to be for compatibility with Paper,
but that it would actually throw an error in that case.

In Paper, `stateNode` is an instance of `ReactNativeFiberHostComponent`,
which doesn't have a `canonical` field. We try to access nested
properties in that field in a couple of places here, which would throw a
type error (cannot read property `_nativeTag` of `undefined`) if we
actually happened to pass a reference to a Paper state node.

In this line:

```javascript
const isFabric = !!(
      fromOrToStateNode && fromOrToStateNode.canonical._internalInstanceHandle
    );
```

If it wasn't Fabric, `fromOrToStateNode.canonical` would be undefined,
and we don't check for that before accessing
`fromOrToStateNode.canonical._internalInstanceHandle`. This means that
we actually never use this logic in Paper or we would've seen the error.

## How did you test this change?

Existing tests.

packages/react-native-renderer/src/ReactFabricGlobalResponderHandler.js

@@ -7,42 +7,24 @@
  * @flow
  */
 
-// Module provided by RN:
-import {UIManager} from 'react-native/Libraries/ReactPrivate/ReactNativePrivateInterface';
-
 const ReactFabricGlobalResponderHandler = {
   onChange: function (from: any, to: any, blockNativeResponder: boolean) {
-    const fromOrTo = from || to;
-    const fromOrToStateNode = fromOrTo && fromOrTo.stateNode;
-    const isFabric = !!(
-      fromOrToStateNode && fromOrToStateNode.canonical._internalInstanceHandle
-    );
-
-    if (isFabric) {
-      if (from) {
-        // equivalent to clearJSResponder
-        nativeFabricUIManager.setIsJSResponder(
-          from.stateNode.node,
-          false,
-          blockNativeResponder || false,
-        );
-      }
+    if (from) {
+      // equivalent to clearJSResponder
+      nativeFabricUIManager.setIsJSResponder(
+        from.stateNode.node,
+        false,
+        blockNativeResponder || false,
+      );
+    }
 
-      if (to) {
-        // equivalent to setJSResponder
-        nativeFabricUIManager.setIsJSResponder(
-          to.stateNode.node,
-          true,
-          blockNativeResponder || false,
-        );
-      }
-    } else {
-      if (to !== null) {
-        const tag = to.stateNode.canonical._nativeTag;
-        UIManager.setJSResponder(tag, blockNativeResponder);
-      } else {
-        UIManager.clearJSResponder();
-      }
+    if (to) {
+      // equivalent to setJSResponder
+      nativeFabricUIManager.setIsJSResponder(
+        to.stateNode.node,
+        true,
+        blockNativeResponder || false,
+      );
     }
   },
 };