d3d11: Revert staging buffer pool implementation

d3d11: Revert staging buffer pool implementation

The staging buffer pool implementation was added to improve
throughput performance since we can avoid per-frame
CPU copy operation via staging texture but it turned out that
we can not make it thread safe. See
https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/1182

Reverting the staging texture implementation as it does not show
any visible value.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2651>