[core][experimental] Remove separate asyncio path for accelerated DAGs

[stephanie-wang]

Description

Right now asyncio support in accelerated DAGs uses a slightly different codepath from normal DAGs:

• you need to specify enable_asyncio=True in dag.experimental_compile()
• you need to use dag.execute_async
• you need to await twice, once for the execution and again for the output

This is because asyncio execution currently starts a background thread to read/write channels, so we need these extra APIs for coordinating with the background thread. With timeout support for channel reads and writes (#46259), we might be able to remove some of these APIs and support asyncio with single-threaded execution, using mostly the same codepath as non-asyncio execution.

Use case

No response

[rkooo567]

downgrade for now. let's revisit after resolving p1s.