[css-forms-1] Fix radio button checkmark styles (#13566)

The original styles always set a checkmark symbol for content, but this leads to it rendering for checked radios too.

Author: lukewarlow

css-forms-1/Overview.bs

@@ -886,11 +886,14 @@ input[type=radio] {
     justify-content: center;
 }
 
-input[type=checkbox]:not([switch])::checkmark,
-input[type=radio]::checkmark {
+input[type=checkbox]:not([switch])::checkmark {
     content: '\2713' / '';
 }
 
+input[type=radio]::checkmark {
+    content: '';
+}
+
 input[type=checkbox]:not([switch], :checked)::checkmark {
 input[type=radio]:not(:checked)::checkmark {
     visibility: hidden;