radv: remove useless sync before CmdClear{Color,DepthStencil}Image()

radv: remove useless sync before CmdClear{Color,DepthStencil}Image()

We don't need to flush anything before these two commands as well.
This is because they have to be externally synchronized, so the
app should have called CmdPipelineBarrier() prior to that and the
driver should have flushed the caches.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>