Drop `async_add_import_executor_job`

[puddly]

await gateway.shutdown() did not actually wait for the import thread to exit properly because I think the code from Core was written to prevent deadlocks at process shutdown. This fixes the failing core/zha unit test that was triggered by reloading zigpy/zha too quickly and shutting down the test.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.84%. Comparing base (9831096) to head (6c485c8).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #55 +/- ## ========================================== - Coverage 94.89% 94.84% -0.05% ========================================== Files 61 61 Lines 9121 9047 -74 ========================================== - Hits 8655 8581 -74 Misses 466 466 ```

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