qt: remove obsolete `qt_intf_t::voutWindowType`
... and only consider video-wallpaper if
platform is windows.
We can not set an appropriate voutWindowType
during initialization, because it depends on
qt-compositor which is evaluated later on.
For example, platform windows might use
both dcomp and hwnd window types. In fact
none of the compositor integrations care about
qt_intf_t::voutWindowType but rather set the
valid type themselves.
Fo example, compositor_dcomp already uses
VLC_WINDOW_TYPE_DCOMP and compositor_platform
already uses VLC_WINDOW_TYPE_HWND, effectively
making qt_intf_t::voutWindowType obsolete.