These were only referenced by the StateManager11. Instead of using
these cache variables, ensure the StateManager11 marks the RT as
dirty and apply the RenderTarget state.

Also remove the RenderTarget setters from Renderer11, and don't
unapply RTs after a one-time set operation. The StateManager11
should take care of updating the necessary changes itself.

BUG=angleproject:1321

Change-Id: Idc8d22ea9e92ac8d07392ab10d067df6084403bb
Reviewed-on: https://chromium-review.googlesource.com/330462Reviewed-by: John Bauman <jbauman@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>