Fix multiple shortcut events being delivered on macOS

Commit ef1d61a3

 introduced a
regression where multiple shortcut events where delivered on
macOS.

Instead of using tryShortcut (which triggers shortcut events) to check if
the shortcut needs to be overridden, check if the shortcut map has an
entry for the key seqeunce in question.

Task-number: QTBUG-66207
Change-Id: I8d277e2ac39de8a1f2d2c43dc8486e042c4ea458
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>