Remove unused router method (#16149)
Remove unused router method (#16149)

This PR removes a legacy router method that was used for old-style HMR, now replaced by Fast Refresh.

This method was not public:
```tsx
export type NextRouter = BaseRouter &
  Pick<
    Router,
    | 'push'
    | 'replace'
    | 'reload'
    | 'back'
    | 'prefetch'
    | 'beforePopState'
    | 'events'
    | 'isFallback'
  >
```

Even if someone found this method, it's highly unlikely they could use it successfully—it required the full module object.
zeit/next.js,flybayer/next.js,flybayer/next.js,azukaru/next.js,JeromeFitz/next.js,azukaru/next.js,flybayer/next.js,JeromeFitz/next.js,azukaru/next.js,JeromeFitz/next.js,flybayer/next.js,JeromeFitz/next.js,azukaru/next.js,zeit/next.js,zeit/next.js