actually delete obsolete qmultimediadefs.h

defining a deprecated forwarding header leads to all kinds of
"interesting" effects when the header is actually still there.

amends 24a1c04b

.

Task-number: QTBUG-58432
Change-Id: I4a32bd55fe7bd14e1aff2f3a6ca46439ce1510de
Reviewed-by: Lars Knoll <lars.knoll@qt.io>