Drop Legacy Edge support.

This allows us to move forward without being held back. Microsoft already replaces the Legacy Edge with the new one on supported Windows versions.

scss/forms/_form-select.scss

@@ -32,16 +32,6 @@
       // Avoid using mixin so we can pass custom focus shadow properly
       box-shadow: $form-select-focus-box-shadow;
     }
-
-    &::-ms-value {
-      // For visual consistency with other platforms/browsers,
-      // suppress the default white text on blue background highlight given to
-      // the selected option text when the (still closed) <select> receives focus
-      // in Edge.
-      // See https://github.com/twbs/bootstrap/issues/19398.
-      color: $input-color;
-      background-color: $input-bg;
-    }
   }
 
   &[multiple],