ColumnView: don't set visible to false on remove

When we set the visibility of the item we removed, the item could still be on the screen, thus lead to unnecessary re-drawing.

ColumnView is used by PageRow internally, if we call pop on PageRow, the current Page's visible will be set to false. If the popped page is a fairly complicate page, for example a ScrollablePage with a large ListView, the UI will freeze for seconds.

Ideally, the visibility should be set to false when it won't trigger re-drawing. But I've yet to find where the place is.

Reproducible example:

import QtQuick 2.15
import QtQuick.Controls 2.15 as QQC2
import QtQuick.Layouts 1.15

import org.kde.kirigami 2.15 as Kirigami
import org.kde.kcm 1.2 as KCM

Kirigami.ApplicationWindow {
    id: root
    pageStack.initialPage: rootPage

    KCM.ScrollViewKCM {
        visible: false
        id: rootPage
        view: ListView {
            model: ["page1", "page2"]
            delegate: Kirigami.BasicListItem {
                highlighted: false
                hoverEnabled: false

                text: modelData
                reserveSpaceForSubtitle: true
                trailing: Item {
                    implicitWidth: changeButton.implicitWidth
                    QQC2.Button {
                        id: changeButton
                        anchors.centerIn: parent
                        text: "goto page"
                        onClicked: {
                            if (modelData === "page1") {
                                root.pageStack.push(page1);
                            } else {
                                root.pageStack.push(page2);
                            }
                        }
                    }
                }
            }
        }


        KCM.ScrollViewKCM {
            id: page1
            title: "page1"
        }


        KCM.ScrollViewKCM {
            id: page2
            title: "page2"
            header: QQC2.Button {
                text: "back"
                onClicked: root.pageStack.pop()
            }

            view: ListView {
                id: localeListView
                clip: true
                model: 1000
                delegate: Kirigami.BasicListItem {
                    icon: "system-reboot"
                }
            }
        }
    }
}

Without this patch, go to page2, click back will freeze the UI for a sec, after apply this patch, the page pops instantly.