Remove ${OBJDUMP} as it is not used by the base system

It was added to sys.mk relatively recently (r274503) for EFI builds
but is no longer used by the base system. The in-tree binutils are
outdated, will not be updated, and will be removed in the future.
Remove it from the toolchain build now to slightly simplify the build
and make sure we don't grow an accidental dependency.

Note that this affects only the toolchain build, and does not affect
/usr/bin/objdump in the built world.

Reviewed by: bdrewery
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D6460