msglist [nfc]: Cut unnecessary condition on bar-background brightness

When the background of stream recipient bars was colored by
`subscription.color` itself, instead of a much-lightened
transformation of that, it was presumably possible that
ThemeData.estimateBrightnessForColor would give Brightness.dark,
making `contrastingColor` be white.

But when we started using
`subscription.colorSwatch().barBackground`, in e97e6fd, the
darkest possible color became #c8c8c8 (a light gray) -- that's from
a base color of true black -- and for that color,
ThemeData.estimateBrightnessForColor gives Brightness.light, making
`contrastingColor` be black. Lighter base colors than true black
will make even lighter `barBackground`s.

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -661,10 +661,7 @@ class StreamMessageRecipientHeader extends StatelessWidget {
     if (subscription != null) {
       final swatch = subscription.colorSwatch();
       backgroundColor = swatch.barBackground;
-      contrastingColor =
-        (ThemeData.estimateBrightnessForColor(swatch.barBackground) == Brightness.dark)
-          ? Colors.white
-          : Colors.black;
+      contrastingColor = Colors.black;
       iconColor = swatch.iconOnBarBackground;
     } else {
       backgroundColor = _kUnsubscribedStreamRecipientHeaderColor;