This function was a duplicate of ValidTexture2DDestinationTarget.
Also clean up a bit of the code in Framebuffer::completeness.

BUG=angle:660

Change-Id: Iee8d011274bfb2e426346e3ccdde7342fbb7ef44
Reviewed-on: https://chromium-review.googlesource.com/202595Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>