dropping the execmodel concept again

CC @ctheune @nicoddemus

the execmodel concept that @hpk42 introduced back in 2013 has multiple drawbacks

1. it breaks execnet for testing qt, as the main thread is no longer under safe control
2. it is completely incompatible with the async primitives that are starting to win (asyncio/trio/curio, async/await)

its not quite clear to me for what detailed use-case it was introduced, but as it is right now its a major show-stopper for me to introduce any support for modern async primitives

its the biggest reason why i dropped execnet maintenances, the second biggest reason being that there seemingly where possible placements (unfortunately the replacements turned out not to work out)

この issue では、execmodel の概念に対応するファイル、テスト、エントリーポイントが示されていません。まず execmodel の実装を見つけて未解決の議論を確認し、その後、変更を試みる前に影響を受ける execnet の動作と互換性要件を定義してください。