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.