Mocha's type definitions pollute and break other node modules 

For example, it changes the return type of `setTimeout` and causes some errors.

```
node_modules/spica/throttle.ts(7,5): error TS2322: Type 'Timer' is not assignable to type 'number'.
node_modules/spica/throttle.ts(28,5): error TS2322: Type 'Timer' is not assignable to type 'number'.
```

This is caused by #22112.

@aozgaa Can you revert that change?