Customizer: If there's only one available sidebar, hide the move-widget button.

props celloexpressions.
fixes #30690.

git-svn-id: https://develop.svn.wordpress.org/trunk@31252 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/js/customize-widgets.js

@@ -641,6 +641,12 @@
 						selectSidebarItem( selfSidebarItem );
 					}
 				} );
+
+				if ( 1 === $sidebarItems.length ) {
+					self.container.find( '.move-widget' ).hide();
+				} else {
+					self.container.find( '.move-widget' ).show();
+				}
 			};
 
 			updateAvailableSidebars();