css: Remove superflous background color on date row.

This fix prevents the active-message outline from being obscured
under certain conditions, such as when setting a browser to zoom
out at 90% or less.

Suggested-by: Anders Kaseorg <anders@zulip.com>

web/styles/message_row.css

@@ -37,7 +37,6 @@ $time_column_max_width: 150px;
 
     .date_row {
         grid-area: date_row;
-        background-color: var(--color-background-stream-message-content);
         /* We only want padding for the date rows between recipient blocks */
         padding-bottom: 0;