The members soft, bleedbias, bleedexp and contact_spread were
deprecated in rBd8aaf25c23fa, and, with the exception of soft,
bleedexp and contact_spread, are no longer used.

soft is only used by Collada as an extra value for exporting and
importing Blender files in collada.

bleedexp and contact_spread are only used in versioning to
initialize a default value.

I'm not sure how backward compatibility will work since these values will no longer be found via DNA_struct_elem_find. But it's not good practice to load new version files into old versions of Blender anyway.

(Patch created in response to D16669#454016)