Optimize does not use the engine import executor anymore

AT:

• optimize does not use engine import executor anymore
• all IT tests are running through
• there configuration settings for the engine executor are removed
• the configuration section in the technical guide is adjusted, such that it does not contain the engine import executor configuration anymore

Context:
This is the first step in enable Optimize to handle the deletion of historic process instances in the engine. For that we can't use the consumer-producer pattern anymore, but should go round robin through all engine endpoints.