drm/vmwgfx: Remove the throttling code

Throttling was used before fencing to implement early vsync
support in the xorg state tracker a long time ago. The xorg
state tracker has been removed years ago and no one else
has ever used throttling. It's time to remove this code,
it hasn't been used or tested in years.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Link: https://patchwork.freedesktop.org/patch/414042/?series=85516&rev=2