Remove archive file in gh-pages branch (#2786)

Summary:
The motivation of generating `artifact.tar.gz` in the `build_docs` job is to easily use it for adding documentation in each stable release. But it is committed into `gh-pages` branch which causes the git repository very huge (see https://github.com/pytorch/audio/issues/2783). This PR removes the tar file from the commit.

Pull Request resolved: https://github.com/pytorch/audio/pull/2786

Reviewed By: carolineechen

Differential Revision: D40591152

Pulled By: nateanl

fbshipit-source-id: 47df60c2ec7bcdcc40e2b6078219b9397e6bfed1

.circleci/build_docs/commit_docs.sh

@@ -20,6 +20,7 @@ git checkout gh-pages
 mkdir -p ./"${target}"
 rm -rf ./"${target}"/*
 cp -r "${src}/docs/build/html/"* ./"$target"
+rm ./"$target"/artifact.tar.gz
 if [ "${target}" == "main" ]; then
     mkdir -p ./_static
     rm -rf ./_static/*