Remove api/python/_sources/

[dongjoon-hyun]

This PR aims to delete api/python/_sources/ to help the on-going 3.5.2 release and the future release like the following.

$ find . -name _sources
./site/docs/4.0.0-preview1/api/python/_sources
./site/docs/3.1.2/api/python/_sources
./site/docs/3.3.1/api/python/_sources
./site/docs/3.3.0/api/python/_sources
./site/docs/3.1.3/api/python/_sources
./site/docs/3.4.0/api/python/_sources
./site/docs/3.2.2/api/python/_sources
./site/docs/3.4.1/api/python/_sources
./site/docs/3.2.4/api/python/_sources
./site/docs/3.2.3/api/python/_sources
./site/docs/3.5.0/api/python/_sources
./site/docs/3.1.1/api/python/_sources
./site/docs/3.3.2/api/python/_sources
./site/docs/3.5.1/api/python/_sources
./site/docs/3.3.3/api/python/_sources
./site/docs/3.3.4/api/python/_sources
./site/docs/3.2.1/api/python/_sources
./site/docs/3.4.3/api/python/_sources
./site/docs/3.2.0/api/python/_sources
./site/docs/3.4.2/api/python/_sources

$ find . -name _sources | xargs rm -rf

$ COLUMNS=1000 git diff HEAD~1 --stat | grep -v '/api/python/_sources'
 35211 files changed, 1263404 deletions(-)