msglist [nfc]: Remove a no-op MainAxisAlignment.spaceBetween in _SenderRow

No-op because the child Flexible -> GestureDetector -> Row has the
default MainAxisSize.max, filling the available space, leaving none
that would be controlled by spaceBetween.

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -1450,7 +1450,6 @@ class _SenderRow extends StatelessWidget {
     return Padding(
       padding: const EdgeInsets.fromLTRB(16, 2, 16, 0),
       child: Row(
-        mainAxisAlignment: MainAxisAlignment.spaceBetween,
         crossAxisAlignment: CrossAxisAlignment.baseline,
         textBaseline: localizedTextBaseline(context),
         children: [