Revert shared taxonomy term splitting for 4.1.

This is a revert of [30494], [30492], [30347], and [30334]. The latter
changeset was a revert of [30336], which was a revert of [30241].

Watch for Term Splitting, Version III in version 4.2, coming soon to a
WordPress trunk near you.

See #30335, #5809.

git-svn-id: https://develop.svn.wordpress.org/trunk@30585 602fd350-edb4-49c9-b593-d223f7449a82