Remove unused batched_lbfgs_b copy (#2953)

Summary:
Pull Request resolved: #2953

The actual batched implementation is available in `botorch/optim/batched_lbfgs_b.py`. This diff removes `botorch/generation/batched_lbfgs_b.py`.

Reviewed By: SamuelGabriel

Differential Revision: D79649660

fbshipit-source-id: 34e8a16fb2051715e1ec562f5afcb3b960255f09

Author: saitcakmak