refactor: remove dead code of fiber (facebook#28389)

The `nextEffect` property of the `fiber` is currently not being used and
has become dead code. It can be safely removed.

Author: bubucuo

packages/react-reconciler/src/ReactInternalTypes.js

@@ -157,9 +157,6 @@ export type Fiber = {
   subtreeFlags: Flags,
   deletions: Array<Fiber> | null,
 
-  // Singly linked list fast path to the next fiber with side-effects.
-  nextEffect: Fiber | null,
-
   // The first and last fiber with side-effect within this subtree. This allows
   // us to reuse a slice of the linked list when we reuse the work done within
   // this fiber.