Port to KStandardAction::MoveToTrash

It was introduced in kconfig(widgets) 5.25. Using the standard action
results in less code and no need to hardcode the Del shortcut.

Test Plan:
- Change shortcut in System Settings -> Shortcuts -> Standard Shortcuts
- Make sure the new shortcut is used by dolphin.

Differential Revision: https://phabricator.kde.org/D6778