chore: remove `on_fork_commit` (#18318)

Simon thinks it was because he thought that committing the fork != committing the batch, and that this could result in an error boundary showing up too late. But that's not the case

packages/svelte/src/internal/client/dom/blocks/boundary.js

@@ -396,7 +396,7 @@ export class Boundary {
 			if (this.#pending_effect) current_batch.skip_effect(this.#pending_effect);
 			if (this.#failed_effect) current_batch.skip_effect(this.#failed_effect);
 
-			current_batch.on_fork_commit(() => {
+			current_batch.oncommit(() => {
 				this.#handle_error(error);
 			});
 		} else {

packages/svelte/src/internal/client/reactivity/batch.js

@@ -140,12 +140,6 @@ export class Batch {
 	 */
 	#discard_callbacks = new Set();
 
-	/**
-	 * Callbacks that should run only when a fork is committed.
-	 * @type {Set<(batch: Batch) => void>}
-	 */
-	#fork_commit_callbacks = new Set();
-
 	/**
 	 * The number of async effects that are currently in flight
 	 */
@@ -634,7 +628,6 @@ export class Batch {
 	discard() {
 		for (const fn of this.#discard_callbacks) fn(this);
 		this.#discard_callbacks.clear();
-		this.#fork_commit_callbacks.clear();
 
 		this.#unlink();
 		this.#deferred?.resolve();
@@ -840,16 +833,6 @@ export class Batch {
 		this.#discard_callbacks.add(fn);
 	}
 
-	/** @param {(batch: Batch) => void} fn */
-	on_fork_commit(fn) {
-		this.#fork_commit_callbacks.add(fn);
-	}
-
-	run_fork_commit_callbacks() {
-		for (const fn of this.#fork_commit_callbacks) fn(this);
-		this.#fork_commit_callbacks.clear();
-	}
-
 	settled() {
 		return (this.#deferred ??= deferred()).promise;
 	}
@@ -1410,10 +1393,6 @@ export function fork(fn) {
 				source.wv = increment_write_version();
 			}
 
-			batch.activate();
-			batch.run_fork_commit_callbacks();
-			batch.deactivate();
-
 			// trigger any `$state.eager(...)` expressions with the new state.
 			// eager effects don't get scheduled like other effects, so we
 			// can't just encounter them during traversal, we need to