remove tools/setup_helpers/cudnn.py (#25482)
Summary:
FindCUDNN.cmake and cuda.cmake have done the detection. This commit deletes `tools/setup_helpers/cudnn.py` as it is no longer needed.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25482
Differential Revision: D17226408
Pulled By: ezyang
fbshipit-source-id: abd9cd0244cabea1f5d9f93f828d632d77c8dd5e
