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.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

[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 #988 (comment)