[cmake] clean up blas discovery (#47940)
Summary:
remove useless variable changes in blas discovery
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47940
Reviewed By: malfet
Differential Revision: D25122228
Pulled By: walterddr
fbshipit-source-id: 12bc3ce9e4f89a72b6a92c10d14024e5941f4b96
