Removing lm_eval as a third-party module (#3793)

Summary:
The third-party module was added when the install requirements wasn't working for lm_eval.

Since the latter was fixed, the module is unused. Harmless removal

Pull Request resolved: https://github.com/pytorch/executorch/pull/3793

Reviewed By: lucylq

Differential Revision: D58037012

Pulled By: Jack-Khuu

fbshipit-source-id: c57b4781d08fa3b4c3a0a3f2eb6de1a6ee3b591d