[v3] traits duplicate removal

[Paolo Carlini]

Hi Doug.

> Looks like some more fallout from this patch...
>
> /Volumes/scratch/dgregor/fsf-gcc-build/powerpc-apple-darwin8.7.0/ 
> libstdc++-v3/include/cmath:52:29: error: ext/type_traits.h: No such  
> file or directory

hummm, this error doesn't make sense for a sane setup... Did you rebuild 
from scratch compiler + library after Benjamin' changes? I did, and that 
file is definitely there for me, everything is fine.

Paolo.