Remove custom executor, use smol's standard multi-threaded executor

[sven-of-cord]

This is another commit in a series of commits to simplify the code base. With the removal of the caching layer and async_memoizer, we no longer spawn tasks in the background. There is no point in having a custom-built executor, just using the built-in one from smol is just fine.

Test Plan: run spr diff or similar command, just to show that the asynchronous code we launch with smol::block_on actually executes.