move_topic_modal: Remove flex properties that caused overflow.

Using flex here caused overflow which was especially visible for
very long channel names(~60 characters). Even for the shorter
channel names there was slight overflow noticeable in input class
move_messages_edit_topic if left and right spacing was compared.

This commit fixes the issue by removing the flex display.

web/styles/popovers.css

@@ -885,10 +885,6 @@ ul.popover-group-menu-member-list {
     }
 
     .topic_stream_edit_header {
-        display: flex;
-        flex-flow: column wrap;
-        justify-content: flex-start;
-
         #select_stream_id {
             border-left: 0;
             padding-left: 0;