plugins/overview: remove middle click to close virtual desktop

This is a hidden destructive action[1] in a context where users are not
primed to expect it. It's also largely unnecessary since the delegates
already show a delete button on hover, and deleting virtual desktops
isn't such a common task that it benefits from an accelerator. Let's
remove it for safety.

[1] It's destructive because user-created state on the deleted virtual
desktop is lost--with no confirmation or undo, to boot.

BUG: 484999
FIXED-IN: 6.1

src/plugins/overview/qml/DesktopBar.qml

@@ -149,17 +149,10 @@ Item {
                         MouseArea {
                             id: mouseArea
                             anchors.fill: parent
-                            acceptedButtons: Qt.LeftButton | Qt.MiddleButton
+                            acceptedButtons: Qt.LeftButton
                             onClicked: mouse => {
                                 mouse.accepted = true;
-                                switch (mouse.button) {
-                                case Qt.LeftButton:
-                                    delegate.activate();
-                                    break;
-                                case Qt.MiddleButton:
-                                    delegate.remove();
-                                    break;
-                                }
+                                delegate.activate();
                             }
                         }