j*huff.c: Remove crufty ASSIGN_STATE() macro

j*huff.c: Remove crufty ASSIGN_STATE() macro

This macro is a relic of libjpeg's historic need to support a wide
variety of C compilers with varying degrees of compatibility.  Such was
necessary during the open systems era, because C compilers were often
supplied by the system vendor.  Prior to 1989, there was no C standard
per se, and even after ANSI C became a thing, there were still compilers
in use that didn't conform to it (libjpeg was first released in 1991.)

Realistically, only a handful of C compilers are in widespread use these
days, and all modern C compilers should support structure assignment.