Remove isPrimaryRenderer from Flight Server Config (#30115)

This was used for Context but since ReactFlightNewContext is gone now
this is no longer used.

packages/react-dom-bindings/src/server/ReactFlightServerConfigDOM.js

@@ -20,10 +20,6 @@ import type {
 // but it does not have any exports
 import './ReactDOMFlightServerHostDispatcher';
 
-// Used to distinguish these contexts from ones used in other renderers.
-// E.g. this can be used to distinguish legacy renderers from this modern one.
-export const isPrimaryRenderer = true;
-
 // We use zero to represent the absence of an explicit precedence because it is
 // small, smaller than how we encode undefined, and is unambiguous. We could use
 // a different tuple structure to encode this instead but this makes the runtime

packages/react-server/src/forks/ReactFlightServerConfig.custom.js

@@ -19,8 +19,6 @@ export type HintCode = any;
 // eslint-disable-next-line no-unused-vars
 export type HintModel<T: any> = any;
 
-export const isPrimaryRenderer = false;
-
 export const supportsRequestStorage = false;
 export const requestStorage: AsyncLocalStorage<Request | void> = (null: any);