Simplify getNearestMountedDOMNode to just walk to path and check the map

We don't need the dependency on findFiberByHostInstance now that we maintain
our own map of public instances.

This relies on the assumption that this is a DOM renderer but we could always
add support for more renderer types that need conflict resolution if they
happen.

packages/react-devtools-shared/src/backend/fiber/renderer.js

@@ -3725,23 +3725,11 @@ export function attach(
   }
 
   function getNearestMountedDOMNode(publicInstance: Element): null | Element {
-    // TODO: Remove dependency on findFiberByHostInstance.
-    const mountedFiber = renderer.findFiberByHostInstance(publicInstance);
-    if (mountedFiber != null) {
-      if (mountedFiber.stateNode !== publicInstance) {
-        // If it's not a perfect match the specific one might be a resource.
-        // We don't need to look at any parents because host resources don't have
-        // children so it won't be in any parent if it's not this one.
-        if (publicInstanceToDevToolsInstanceMap.has(publicInstance)) {
-          return publicInstance;
-        }
-      }
-      return mountedFiber.stateNode;
+    const domNode: null | Element = publicInstance;
+    while (domNode && !publicInstanceToDevToolsInstanceMap.has(domNode)) {
+      domNode = domNode.parentNode;
     }
-    if (publicInstanceToDevToolsInstanceMap.has(publicInstance)) {
-      return publicInstance;
-    }
-    return null;
+    return domNode;
   }
 
   function getElementIDForHostInstance(