This removes some duplicate unreachable calls from TextureD3D_External
and TextureD3D_2DMultisample. There will be further use for this once
TextureD3D_2DMultisampleArray is implemented.

This also cleans up some no-ops from TextureD3D_2DMultisample and sets
it correctly as immutable.

BUG=angleproject:2775
TEST=angle_end2end_tests

Change-Id: Ia253a1ed6f418a24bb0c4a4e61654b1c061dba97
Reviewed-on: https://chromium-review.googlesource.com/1193950Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>