Remove references to SNS

[fureigh]

The app doesn't currently use SNS, but the following files currently contain references to SNS:

• aws.py
• README.rst
• signing.py

For the sake of cleanup and clarity, let's remove those from the codebase.

(🎩 tip to @jmcarp for confirming that AutoAPI isn't currently using SNS.)

For anyone with access to the relevant Slack channel, here's more context.