Delete cuda9-cudnn7 build which is not defined in build.sh (#38750)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/38750
Test Plan: `grep -R pytorch-linux-xenial-cuda9-cudnn7-py3 .circleci`
Differential Revision: D21654262
Pulled By: malfet
fbshipit-source-id: a20cba15e9a24e9cbca7a8111d9149a9ae725886
