Replace map widget logic with MultiValue form element

Problem/Motivation

The MultiValue form element is now robust and has test coverage and therefore can replace the custom code that provides the form logic and callbacks for map_text and map_key_value sub-field widgets. This will allow us to standardize and eliminate unnecessary extra code while providing more reliable function particularly when these widgets are used within other sub-forms and such.

Proposed resolution

Replace custom code with MultiValue form element for both of these widget types.