[Bug]: Different big number libraries being used

### Describe the bug

We have multiple BigNumber libraries in use right now, `bignumber.js`, `bn.js`, etc

You can see this from `app/util/number/index.js`

```
import { BN, stripHexPrefix } from 'ethereumjs-util';
import { utils as ethersUtils } from 'ethers';
import convert from 'ethjs-unit';
import {
BNToHex,
hexToBN as controllerHexToBN,
} from '@metamask/controller-utils';
import numberToBN from 'number-to-bn';
import BigNumber from 'bignumber.js';
```

Would be good to settle on a single library.

### Expected behavior

A singular BigNumber library should be used to prevent incompatibilities or strange/unexpected behavior from happening.

### Screenshots/Recordings

_No response_

### Steps to reproduce

n/a

### Error messages or log output

_No response_

### Version

7.24.0

### Build type

None

### Device

Android, iOS

### Operating system

iOS, Android

### Additional context

_No response_

### Severity

_No response_

Start with app/util/number/index.js and inventory how the listed BigNumber libraries are used across the project. Determine which single library can cover those conversions; done means the number utilities no longer mix incompatible BigNumber implementations.

Written by the indexing model from the issue text.