Retiring WITHOUT_ELFCOPY_AS_OBJCOPY option

[Ed Maste]

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

As part of the plan to remove the outdated in-tree binutils in FreeBSD
12 I intend to remove the WITHOUT_ELFCOPY_AS_OBJCOPY build knob, after
FreeBSD 11.0 is released.

The patch is available in a Phabricator code review:
https://reviews.freebsd.org/D7337