Clean up TextureD3D_2DMultisample 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/1193950 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>