texobj: Remove redundant checks that the texture cube faces match size

texobj: Remove redundant checks that the texture cube faces match size

The texture mipmap completeness checking code was checking whether all
of the faces have the same size. However this is pointless because the
code just above it checks whether the face has the expected size
calculated for the mipmap level anyway so the error condition could
never be reached. This patch just removes it.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>