remove dependency on local_aws

[danmarsden]

it seems to me that there are some useful components to this plugin that would be handy even for non-AWS users - would be good if we could use this plugin without local_aws and just enable the AWS SNS stuff if the plugin exists?

[brendanheywood]

I had that thought when adding dkim, on top of that the aws sdk is going to be in core so local_aws is likely to be completely deprecated at some point, it is already bundled in totara, so it needs to be a soft dependency either way