fix extra keydown handler registration, fixes #17825

The dojox/mobile/ListItem keydown handler is called twice when a key is pressed. This is because the keydown handler is registered twice: first in ListItem.startup, then in ListIem._updateHandles, which is invoked indirectly when ListItem.startup() calls this.inherited().
Since ListItem._updateHandles() is invoked by ItemBase.startup(), this patch removes the useless event registration done in startup().