Editor: Improve edits reducer's unsetting of merged values

[aduth]

Related: #10827 (comment)

While handling of nested values (i.e. meta) was improved in #10827, it left a remaining edge case where updated values may be needlessly left in edits state if not all meta values can be considered as updated to their canonical form.

We should consider improving the reducer such that in the handling of UPDATE_POST, if the key for which updates are received is one of the file's EDIT_MERGE_PROPERTIES properties, it should individually unset values from the received canonical value which align, removing the top-level key if all values are the same.

[paaljoachim]

This issue is now really old and I see a PR has been merged.
I will go ahead and close this issue.
@gziolo (I am pinging you in here Greg.)