setf: excise GET-SETF-METHOD and related detritus.
* GET-SETF-METHOD is the CLtL1 version of GET-SETF-EXPANSION, and is
neither used in SBCL nor exported from any package. Kill it.
* Also kill the DEFKNOWN in the compiler for a (different)
GET-SETF-METHOD and GET-SETF-METHOD-MULTIPLE-VALUE (also a CLtL1
function, neither used nor implemented in SBCL).