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 worked since
SBCL-1.1.17, 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.