remove text allowing UA tweaks to author highlight colors

Author: delan

css-pseudo-4/Overview.bs

@@ -796,13 +796,6 @@ Highlight Colors</h4>
   when the author sets one “custom” highlight color,
   defaults for the other highlight color are suppressed.
 
-  This does not preclude the UA
-  from transforming “custom” highlight colors
-  in some or all situations.
-  For example,
-  UAs may invert highlight colors based on similarity with nearby colors,
-  or vary their opacities.
-
   Note: 'stroke-color' and 'fill-color' have their usual effects
   on the foreground color,
   but unlike 'color',