Revert "Issue #2002336 by mgifford, falcon03, Liam Morland: Introduce a class...

Revert "Issue #2002336 by mgifford, falcon03, Liam Morland: Introduce a class to hide borders fieldsets elements."

This is not in-line with CSS standards, so temporarily rolling back for
further discussion.

This reverts commit 20faa056.

core/modules/system/css/system.theme.css

@@ -99,13 +99,6 @@ abbr.form-required, abbr.tabledrag-changed, abbr.ajax-changed {
   margin-left: 0;
   margin-right: 0;
 }
-fieldset.fieldset-no-border {
-  border: none;
-  padding: 0;
-}
-fieldset.fieldset-no-border legend {
-  text-transform: none;
-}
 
 /**
  * Inline items.