refactor(backend): remove unnecessary isNotNull sugar

packages/backend/src/misc/is-not-null.ts

@@ -1,8 +0,0 @@
-/*
- * SPDX-FileCopyrightText: syuilo and misskey-project
- * SPDX-License-Identifier: AGPL-3.0-only
- */
-
-export function isNotNull<T extends NonNullable<unknown>>(input: T | undefined | null): input is T {
-	return input != null;
-}