Remove `classes_excluded_from_instrumentation_default`?

I think `classes_excluded_from_instrumentation_default` does more harm than good. It doesn’t seem to reduce the startup overhead and it can lead to surprises when trying to instrument something that has been globally excluded.

Let's remove it if there are no objections.

See https://github.com/elastic/apm-agent-java/pull/1920#issuecomment-905519224 and https://github.com/elastic/apm-agent-java/pull/1920#issuecomment-906475452 for more details.

Relates to #1920