backend: remove unused prelude modules

Much of these modules are no longer used in the backend. They seem to be
from before the code was organized in packages.

packages/backend/src/prelude/math.ts

@@ -1,3 +0,0 @@
-export function gcd(a: number, b: number): number {
-	return b === 0 ? a : gcd(b, a % b);
-}

packages/backend/src/prelude/maybe.ts

@@ -1,20 +0,0 @@
-export interface IMaybe<T> {
-	isJust(): this is IJust<T>;
-}
-
-export interface IJust<T> extends IMaybe<T> {
-	get(): T;
-}
-
-export function just<T>(value: T): IJust<T> {
-	return {
-		isJust: () => true,
-		get: () => value,
-	};
-}
-
-export function nothing<T>(): IMaybe<T> {
-	return {
-		isJust: () => false,
-	};
-}

packages/backend/src/prelude/string.ts

@@ -1,15 +0,0 @@
-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/test/prelude/maybe.ts

@@ -1,18 +0,0 @@
-import * as assert from 'assert';
-import { just, nothing } from '../../src/prelude/maybe.js';
-
-describe('just', () => {
-	it('has a value', () => {
-		assert.deepStrictEqual(just(3).isJust(), true);
-	});
-
-	it('has the inverse called get', () => {
-		assert.deepStrictEqual(just(3).get(), 3);
-	});
-});
-
-describe('nothing', () => {
-	it('has no value', () => {
-		assert.deepStrictEqual(nothing().isJust(), false);
-	});
-});