Remove renaming of "name" parameter

Seems like there is no way to pass a name to each subimage's header when writing multipart exrs.
The name attribute of each ImageSpec seems a good place to pull that from, but it's currently being remapped to "oiio::subimagename" when writing to the subimage's header.
Each subimage then defaults to being named "subimagexx".

Is there a practical reason for this? Or a different way to set the header's name that I'm missing?

Thanks