x86-64: Remove not-quite-working !N-FIXNUM-TAG-BITS-CASE

This was supposed to elide the code for un-needed values of
n-fixnum-tag-bits in the target, but because of how a VOP-PARSE
stores its body as a literal sexpr, it doesn't do what I mean.

Also, SET-UP-FUN-TRANSLATION isn't bootstrap-only.