Remove optgroup resets from #7611 since Normalize fixes those upstream

dist/css/bootstrap.css

@@ -1669,11 +1669,6 @@ select[multiple],
 select[size] {
   height: auto;
 }
-select optgroup {
-  font-family: inherit;
-  font-size: inherit;
-  font-style: inherit;
-}
 input[type="file"]:focus,
 input[type="radio"]:focus,
 input[type="checkbox"]:focus {

less/forms.less

@@ -72,13 +72,6 @@ select[size] {
   height: auto;
 }
 
-// Fix optgroup Firefox bug per https://github.com/twbs/bootstrap/issues/7611
-select optgroup {
-  font-size: inherit;
-  font-style: inherit;
-  font-family: inherit;
-}
-
 // Focus for file, radio, and checkbox
 input[type="file"]:focus,
 input[type="radio"]:focus,