fix: Remove async example

[e-carlin]

Fixes #988

The async example never actually worked in an async way. It tricked users into thinking they were using async code but in reality the sendgrid_client.send call (which is what was waiting on I/O) was still blocking.

In addition, asyncio.async (used in the example) was deprecated in Python 3.7 and removed in Python 3.10.

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[RobertoPrevato]

I don't think that removing the async example is a good idea. It would be best to acknowledge that currently the SDK does not support async and offer an example that uses the REST API. FWIW, I prepared an async example that uses the REST API directly rather than the SDK https://github.com/sendgrid/sendgrid-python/issues/988#issuecomment-2794847275