Allow users to remove existing terms when adding new terms

[stevejburge]

One user of the "Existing Content" feature wants a new option:

"I used to use "Automatic Post Tagger" which has an option to remove tags while retagging, shown here:"

[reekes]

that use is me ;)

I find this is needed in a few use cases. Without first removing existing tags there's no way within TaxoPress to get a clean start. I've been optimizing my entire taxonomy, but TaxoPress only add new tags. We need a way to remove existing tags, such as the tags that no longer match.