Add 'Clear Value' option to tag editor

[RJWerning]

Is your feature request related to a problem? Please describe.
While tagging values during custom training, on occasion you can get into a situation where the value of a field is just wrong and the only way out of it is to clear the tagged value and start over.

Describe the solution you'd like
Add an option to the tag dropdown (below delete tag?) that allows the value of that field for the document you are training to be cleared out.

Describe alternatives you've considered
Since there is no 'Clear value' option, I resorted to deleting the xxx.labels.json file in blob storage and starting that file over.