deps: drop aiofiles dependency

[oliver-sanders]

We are using aiofiles to facilitate asynchronous IO.

The way aiofiles achieves this is by wrapping IO functions with a decorator which causes them to be run in a ThreadPoolExecutor. The decorator they use can be found here:

https://github.com/Tinche/aiofiles/blob/213ee8b325e09bb6026c2495cf5810023f342f3f/src/aiofiles/ospath.py#L7-L15

In Cylc we use a similar decorator to wrap a couple of functions:

https://github.com/cylc/cylc-flow/blob/994b774f9562f8ee606c37c3ffe1a76df0e5f61b/cylc/flow/async_util.py#L463-L477

This avoids the need for the aiofiles dependency.