Don't hide control box's close button in mobile view.

There are cases where one would like to show it and the sticky_controlbox
config option can be used to remove it.

css/converse.css

@@ -1536,12 +1536,6 @@
       #conversejs #controlbox .controlbox-head {
         border-top-left-radius: 0;
         border-top-right-radius: 0; } }
-    @media screen and (max-height: 450px) {
-      #conversejs #controlbox .controlbox-head .chatbox-btn.close-chatbox-button {
-        display: none; } }
-    @media screen and (max-width: 480px) {
-      #conversejs #controlbox .controlbox-head .chatbox-btn.close-chatbox-button {
-        display: none; } }
   #conversejs #controlbox form.search-xmpp-contact {
     margin: 0;
     padding-left: 5px;

sass/_controlbox.scss

@@ -24,16 +24,6 @@
             height: $chat-head-height;
             margin: 0;
             padding: 6px 6px 6px 0;
-
-            .chatbox-btn.close-chatbox-button {
-                @media screen and (max-height: $mobile-landscape-height) {
-                    display: none;
-                }
-                @media screen and (max-width: $mobile-portrait-length) {
-                    display: none;
-                }
-            }
-
         }
         form.search-xmpp-contact {
             margin: 0;