Makefile.amd64: remove construct that serves no purpose

The variable expansion as written will never match anything but
'amd64/.8' in OBJDIR. The original intention behind the construct
remains unclear, but "as is" it serves no other purpose but to
generate the warning. Remove it altogether.

Fixes: df90aeac24c9
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D38440