left_sidebar: Remove padding from bot icon.

This is no longer necessary for vertical alignment, and by
removing the 6px of padding it represents, bot DM rows will have
the same 22px height as other DM rows.

(cherry picked from commit ea83b1f4de)

web/styles/left_sidebar.css

@@ -1111,10 +1111,6 @@ li.topic-list-item {
 
     .zulip-icon.zulip-icon-bot {
         font-size: 90%; /* Reduce the bulkiness of this icon */
-        /* TODO: Eliminate this top and bottom padding once
-           line-heights have been better established in the
-           DM rows. */
-        padding: 3px 0;
     }
 
     .unread_count {