Remove unused BigInt extension (#5447)

packages/types-codec/src/abstract/BigInt.ts

@@ -1,16 +0,0 @@
-// Copyright 2017-2023 @polkadot/types-codec authors & contributors
-// SPDX-License-Identifier: Apache-2.0
-
-import { BigInt } from '@polkadot/x-bigint';
-
-// @ts-expect-error not extensible, we make it extensible here
-export abstract class AbstractBigInt extends BigInt {
-  // @ts-expect-error super() not required, the self magic does that
-  constructor (value: string | number | bigint | boolean) {
-    const self = Object(BigInt(value)) as typeof AbstractBigInt;
-
-    Object.setPrototypeOf(self, AbstractBigInt.prototype);
-
-    return self;
-  }
-}