Removes support for fallbacks in D3D9 texture formats. The fallback
formats did not work properly anyways.

BUG=angle:658

Change-Id: Idfa5183bf71fd8ebf4608f940f9d93177b9eff08
Reviewed-on: https://chromium-review.googlesource.com/200813Tested-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>