remove unused function

not present upstream, not mentioned anywhere

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

@@ -104,7 +104,3 @@ export function toArray<T>(x: T | T[] | undefined): T[] {
 export function toSingle<T>(x: T | T[] | undefined): T | undefined {
 	return Array.isArray(x) ? x[0] : x;
 }
-
-export function toSingleLast<T>(x: T | T[] | undefined): T | undefined {
-	return Array.isArray(x) ? x.at(-1) : x;
-}