Retire WITHOUT_ELFCOPY_AS_OBJCOPY option

In FreeBSD 11 ELF Tool Chain's elfcopy is installed as objcopy by
default, with the option to switch back to GNU objcopy by setting
WITHOUT_ELFCOPY_AS_OBJCOPY in make.conf.

We plan to remove the outdated in-tree binutils in FreeBSD 12, so
remove the temporary transition aid.

Reviewed by: brooks, imp
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D7337