ActionMenuItem: Shorten one binding expression, remove useless check

Checking for existence of an attached property in QML is pointless due to the fact that engine would always construct an attached object the first time it's accessed.

Also, it's quite strange that any of this works with 2.3 version of QQC2 import, as documentation claims that ActionGroup is Since: Qt 5.10. But I checked it on a separate isolated file, and even normal & attached forms of object declaration worked fine.