tools: bpftool: Cleanup license mess

Precise and non-ambiguous license information is important. The recent
relicensing of the bpftools introduced a license conflict.

The files have now:

     SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause

and

     * This program is free software; you can redistribute it and/or
     * modify it under the terms of the GNU General Public License
     * as published by the Free Software Foundation; either version
     * 2 of the License, or (at your option) any later version

Amazingly about 20 people acked that change and neither they nor the
committer noticed. Oh well.

Digging deeper: The files were imported from the iproute2 repository with
the GPL V2 or later boiler plate text in commit b66e907c ("tools:
bpftool: copy JSON writer from iproute2 repository")

Looking at the iproute2 repository at

  git://git.kernel.org/pub/scm/network/iproute2/iproute2.git

the following commit is the equivivalent:

  commit d9d8c839 ("json_writer: add SPDX Identifier (GPL-2/BSD-2)")

That commit explicitly removes the boiler plate and relicenses the code
uner GPL-2.0-only and BSD-2-Clause. As Steven wrote the original code and
also the relicensing commit, it's assumed that the relicensing was intended
to do exaclty that. Just the kernel side update failed to remove the boiler
plate. Do so now.

Fixes: 907b2236

 ("tools: bpftool: dual license all files")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Jakub Kicinski <jakub.kicinski@netronome.com>
Cc: Roman Gushchin <guro@fb.com>
Cc: YueHaibing <yuehaibing@huawei.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: Stanislav Fomichev <sdf@google.com>
Cc: Sean Young <sean@mess.org>
Cc: Jiri Benc <jbenc@redhat.com>
Cc: David Calavera <david.calavera@gmail.com>
Cc: Andrey Ignatov <rdna@fb.com>
Cc: Joe Stringer <joe@wand.net.nz>
Cc: David Ahern <dsahern@gmail.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Petar Penkov <ppenkov@stanford.edu>
Cc: Sandipan Das <sandipan@linux.ibm.com>
Cc: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Taeung Song <treeze.taeung@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Quentin Monnet <quentin.monnet@netronome.com>
CC: okash.khawaja@gmail.com
Cc: netdev@vger.kernel.org
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>