Revert "widgetexplorer: Delete newStuffDialog on closed"

This reverts commit ee1cd1ab.

There will be a crash after the dialog is opened for the second time if
an installation is in progress, because the dialog component is not
deleted after QtQuickDialogWrapper is destructed.

CCBUG: 452865
2 jobs for Plasma/5.24 in 5 minutes and 45 seconds (queued for 2 seconds)