Remove leftover sub-tag support.
Merge branch 'master' of github.com:discourse/discourse
Revert "Merge branch 'onebox-safety' of git://github.com/tms/discourse"

This reverts commit 7ca57db97afda4d1c130269061f0db3cb9d0dd42, reversing
changes made to b7e027cfd1590365ac9c85141ff2b02c25e900fb.