Remove expando that we were adding to portal container

**what is the change?:**
see title

**why make this change?:**
this is part of an old approach to detecting portals, and we have
instead added a check in the `findHostInstance` method to filter out
portals.

**test plan:**
`yarn test`

**issue:**
facebook#8854

src/renderers/shared/fiber/ReactFiberCommitWork.js

@@ -218,10 +218,6 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
           }
         } else {
           if (isContainer) {
-            if (parentFiber.tag === HostPortal) {
-              // allows us to identify the portal container in other places
-              parent.__reactInternalIsPortalContainer = true;
-            }
             appendChildToContainer(parent, node.stateNode);
           } else {
             appendChild(parent, node.stateNode);