Remove executor config pickling -- and make note of other pickling usages as well

### Body

Shall we remove pickling of executor config?

It's a bit of a breaking change but, who uses that anyway?

While you're at it, please take a look and see if there's anything else that we are pickling that we could ... stop pickling (such as xcom etc) and please create issues for that as well / bring to dev list.

### Committer

- [x] I acknowledge that I am a maintainer/committer of the Apache Airflow project.

Start by locating how executor configuration is pickled and tracing other pickling usages, including XCom. Determine the compatibility impact of removing executor-config pickling, then document other candidates and create follow-up issues or bring them to the dev list. Done means the executor-config decision is implemented and the remaining usages are recorded for separate discussion.

Written by the indexing model from the issue text.