Fix #11073

http://musescore.org/en/node/11073

The Score::deleteItem() implementation already does that: after deleting a note from a chord, another note - specifically the chord->downNote() - should be automatically selected.

But later in the Score::cmdDeleteSelection() method all the selection was cleared, unless the user is in Note Entry mode. I don't know what was the intention of this block of code, but removing it solves the bug and (as far as I can tell) there is no negative side-effect.