binutils: disconnect objdump from the build

The in-tree binutils is old and will not be updated. It does not support
all archs supported by FreeBSD, and for the archs it does support not all
CPU features are supported.

Other tools have migrated to copyfree alternatives. Although llvm-objdump
is nearly a drop-in replacement for GNU objdump it is missing a few options
and has some differences in output format. For now just remove GNU objdump;
ports and developers can use a contemporary, maintained version from ports
or packages. We can revisit installing llvm-objdump as objdump in the
future.

PR: 212319 [exp-run]
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D7338