removed outdated formatting of date strings

[matthewhegarty]

Problem

widget module no longer needs to check for code pre django 3.* as it is now no longer supported.

Solution

• Removed outdated logic

Acceptance Criteria

• Test suite

[coveralls]

coverage: 100.0% (+0.05%) from 99.952%
when pulling 91f3326 on matthewhegarty:remove-date-format-logic
into 31524a4 on django-import-export:main.