Remove automatic fetch cache instrumentation (facebook#28896)

This removes the automatic patching of the global `fetch` function in
Server Components environments to dedupe requests using `React.cache`, a
behavior that some RSC framework maintainers have objected to.

We may revisit this decision in the future, but for now it's not worth
the controversy.

Frameworks that have already shipped this behavior, like Next.js, can
reimplement it in userspace.

I considered keeping the implementation in the codebase and disabling it
by setting `enableFetchInstrumentation` to `false` everywhere, but since
that also disables the tests, it doesn't seem worth it because without
test coverage the behavior is likely to drift regardless. We can just
revert this PR later if desired.

packages/react/src/ReactServer.experimental.js

@@ -7,9 +7,6 @@
  * @flow
  */
 
-// Patch fetch
-import './ReactFetch';
-
 export {default as __SERVER_INTERNALS_DO_NOT_USE_OR_WARN_USERS_THEY_CANNOT_UPGRADE} from './ReactSharedInternalsServer';
 
 import {forEach, map, count, toArray, only} from './ReactChildren';

packages/react/src/ReactServer.js

@@ -7,9 +7,6 @@
  * @flow
  */
 
-// Patch fetch
-import './ReactFetch';
-
 export {default as __SERVER_INTERNALS_DO_NOT_USE_OR_WARN_USERS_THEY_CANNOT_UPGRADE} from './ReactSharedInternalsServer';
 
 import {forEach, map, count, toArray, only} from './ReactChildren';