Remove support for struct definitions in SBCL.

This feature attempted to store/restore entire struct
definitions, including :constructor, :copier, etc. It used
internal symbols from SB-KERNEL and had not quite worked for
years, producing a failure in tests.lisp. With the next SBCL
release it doesn't even compile.

cl-store had only offered this feature with SBCL and CMUCL;
now only CMUCL.

Struct instances continue to be supported, of course.