remove unused files

packages/backend/src/misc/prelude/math.ts

-/*
- * SPDX-FileCopyrightText: syuilo and misskey-project
- * SPDX-License-Identifier: AGPL-3.0-only
- */
-
-export function gcd(a: number, b: number): number {
-	return b === 0 ? a : gcd(b, a % b);
-}

packages/backend/src/misc/prelude/string.ts

-/*
- * SPDX-FileCopyrightText: syuilo and misskey-project
- * SPDX-License-Identifier: AGPL-3.0-only
- */
-
-export function concat(xs: string[]): string {
-	return xs.join('');
-}
-
-export function capitalize(s: string): string {
-	return toUpperCase(s.charAt(0)) + toLowerCase(s.slice(1));
-}
-
-export function toUpperCase(s: string): string {
-	return s.toUpperCase();
-}
-
-export function toLowerCase(s: string): string {
-	return s.toLowerCase();
-}

packages/backend/src/misc/prelude/symbol.ts

-/*
- * SPDX-FileCopyrightText: syuilo and misskey-project
- * SPDX-License-Identifier: AGPL-3.0-only
- */
-
-export const fallback = Symbol('fallback');