Modify dropdowns to be able to remove selected value when field is not mandatory

Some dropdowns (ones with default select html tag, not ng-select) don't allow removing the selected value, which is a problem for fields that are not mandatory.

No files or tests are named. Start by locating the default HTML select dropdowns and comparing their clearing behavior with the ng-select controls; done means an optional dropdown can remove its selected value while mandatory fields retain their required behavior.

Written by the indexing model from the issue text.