[Flight] Delete Server Context (#28225)

Server Context was never documented, and has been deprecated in
#27424.

This PR removes it completely, including the implementation code.

Notably, `useContext` is removed from the shared subset, so importing it
from a React Server environment would now should be a build error in
environments that are able to enforce that.

packages/react-client/src/ReactFlightClient.js

@@ -48,8 +48,6 @@ import {
   REACT_POSTPONE_TYPE,
 } from 'shared/ReactSymbols';
 
-import {getOrCreateServerContext} from 'shared/ReactServerContextRegistry';
-
 export type {CallServerCallback};
 
 type UninitializedModel = string;
@@ -634,10 +632,6 @@ function parseModelString(
         // Symbol
         return Symbol.for(value.slice(2));
       }
-      case 'P': {
-        // Server Context Provider
-        return getOrCreateServerContext(value.slice(2)).Provider;
-      }
       case 'F': {
         // Server Reference
         const id = parseInt(value.slice(2), 16);