Remove some dead code from DDB's amd64 stack unwinder.

The amd64 port copied some code from i386 to fetch function arguments and
display them in backtraces. However, it was commented out and can't easily
be implemented since the function arguments are passed in
registers rather than on the stack in amd64. Remove it in preparation for
some bug fixes in this area.

Reviewed by: jhb
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D2857