How to hide label in customized contact form?
The Display settings are for the message that is sent.  Setting label visibility in this section would impact how it appears in the email body.

The Manage Form Display are the settings for the form - unfortunately there are no options for label display.  You can alter the form by implementing hook_form_alter() https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Form!form.api.php/function/hook_form_alter/8.2.x  OR if you just want to hide that label you could do it with CSS in your theme, a simple display: none; should work.