[CI] Delete `nnpack` installation from conda (#87813)
Not sure why it was there to begin with and I really hope none of our CI depend on the package that was last updated 5 years ago, see https://anaconda.org/killeent/nnpack
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87813
Approved by: https://github.com/atalman, https://github.com/kit1980, https://github.com/ZainRizvi
