Removed -mediaplayer-backend configure param.
It allows to build directshow media player together with wmf's one,
if the last is available.

It is useful to switch to another backend without rebuilding.

By default directshow media player will be used.

[ChangeLog][Platform Specific Changes][Windows] Removed
-mediaplayer-backend config param which allowed to build wmf
together with DirectShow.

Task-number: QTBUG-32783
Change-Id: I486a93d9493407af3793cfe338b35daab7c642db
Reviewed-by: Andy Shaw <andy.shaw@qt.io>