Matches the default-config removal in the preceding commit.
default_toolsets was documented for users to set but was never actually
read at runtime, so showing it in the example config and the delegation
user guide was misleading.
No deprecation note is added: the key was always a no-op, so users who
copied it from the example continue to see no behavior change. Their
config.yaml still parses; the key is just silently unused, same as
before.
Part of Initiative 2 / M0.5.
baaf49e9fd