Remove close button from extension panel

As a result of CHANNELMGR-2098 also the button of the Extension panel in the right side panel must be updated. The Close button from the bottom must be removed, only the X button on top should close the panel.

Also check if the ESC button works properly to close the panel.

Check: is the X-button from the right side panel used by the extension panel? Or does it define its own? (not sure). If it has its own: remove, and use the one from the right side panel.