Revert "Doc server: support interfaces in src/extensions"

The docserver is timing out and it has something to do with this. Note
that as part of the rollback I need to make sure that app.yaml and
cron.yaml still increase their versions.

BUG=350640
TBR=rockot@chromium.org

Review URL: https://codereview.chromium.org/191853002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255741 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255741