How to turn fieldset to normal field for date field
Drupal 8 core date field looks like this .... 





perhaps your theme is overriding this field type like boostrap or some thing if that is the case find the template file move to your sub theme and change it back to normal.

UPDATE: 

But If you are using the Select Widget You have some options:

1) Make css to fix the look of it, a good fast option all you need to do is find the field ID then change the rules for that id only. if not using a custom theme you will need a form alter and attach a custom css library. 

2) Override the Field set template if you turn on theme debugging (https://www.drupal.org/docs/8/theming/twig/debugging-twig-templates)
and inspect the element will see the templates that the field uses, you can copy them in to your theme and override them, or create your own theme hooks see my example of hook_theme_suggestions_HOOK_alter on https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21theme.api.php/function/hook_theme_suggestions_HOOK_alter/8.2.x