Don't split shared terms on term update.

Splitting shared terms means assigning a new term_id to a given term_taxonomy_id.
It was uncovered that this change could cause problems for sites that have
cached the original term_id somehow - say, in postmeta - since future lookups
using that term_id will now fail.

Removing for 4.1-beta1. We'll look at improvements to backward compatibility
to try to get this back into a later beta.

Props mboynes.
See #30335.

git-svn-id: https://develop.svn.wordpress.org/trunk@30336 602fd350-edb4-49c9-b593-d223f7449a82