Fix cleanup of forced window shortcuts

XdgToplevelWindow doesn't call finishWindowRules(). It creates a problem
for Workspace::removeWindow() because it calls setShortcut() to release
the window shortcut.

While one way to fix the bug would be to add a finishWindowRules() call
in XdgToplevelWindow, it would perhaps be not the best one because it
would change the appearance of decoration when the window is closed.

Instead, this change makes the workspace release the shortcut when the
window is closed. It has a couple of advantages: the appearance of the
decoration won't change, shortcut cleanup is better encapsulated.

BUG: 478647


(cherry picked from commit b98607c6)