exec: rm --no-subreaper flag
exec: rm --no-subreaper flag

This was removed from runc exec by commit f61c6e413f0 about 5 years ago,
so it's time to remove it entirely.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

kolyshkin/runc,rhatdan/runc,kolyshkin/runc,kolyshkin/runc,rhatdan/runc,rhatdan/runc,dqminh/runc,dqminh/runc,opencontainers/runc,dqminh/runc,opencontainers/runc,kolyshkin/runc,rhatdan/runc,opencontainers/runc,opencontainers/runc,dqminh/runc