chore: remove unused function (#10762)

Co-authored-by: Rich Harris <rich.harris@vercel.com>

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

@@ -128,19 +128,6 @@ export function mutate_store(store, expression, new_value) {
 	return expression;
 }
 
-/**
- * @template V
- * @param {unknown} val
- * @returns {val is import('#client').Store<V>}
- */
-export function is_store(val) {
-	return (
-		typeof val === 'object' &&
-		val !== null &&
-		typeof (/** @type {import('#client').Store<V>} */ (val).subscribe) === 'function'
-	);
-}
-
 /**
  * @param {import('#client').Store<number>} store
  * @param {number} store_value