Disable moveBefore experiment (facebook#33348)

There seems to be some bugs still to work out in Chrome. See facebook#33187.

Additionally, since you can't really rely on this function existing
across browsers, it's hard to depend on its behavior anyway. In fact,
you now have a source of inconsistent behaviors across browsers to deal
with.

Ideally it would also be more widely spread in fake DOM implementations
like JSDOM so that we can use it unconditionally. facebook#33177.

We still want to enable this since it's a great feature but maybe not
until it's more widely available cross-browsers with fewer bugs.

Author: sebmarkbage

packages/shared/ReactFeatureFlags.js

@@ -194,7 +194,7 @@ export const disableLegacyContext = true;
 export const disableLegacyContextForFunctionComponents = true;
 
 // Enable the moveBefore() alternative to insertBefore(). This preserves states of moves.
-export const enableMoveBefore = __EXPERIMENTAL__;
+export const enableMoveBefore = false;
 
 // Disabled caching behavior of `react/cache` in client runtimes.
 export const disableClientCache = true;