Port to KStandardAction::DeleteFile

Summary:
It was introduced in kconfig 5.25 and it properly handles the shift+del
shortcut (together with kxmlgui >= 5.30). This allows us to drop the
custom delete shortcut as well as the shift+del workaround in
DolphinMainWindow.

Test Plan: Shift+Del still deletes files, without the 'ambiguous shortcut' warning dialog.

Reviewers: emmanuelp

Differential Revision: https://phabricator.kde.org/D5010