Remove the session GC task and API

Since 2017-02 dynamodb now can automatically and freely run garbage collection on tables. It's more cost effective and have no performance impact on sites using dynamodb as a session storage.

source: https://aws.amazon.com/about-aws/whats-new/2017/02/amazon-dynamodb-now-supports-automatic-item-expiration-with-time-to-live-ttl/

Update your project with composer require "silverstripe/dynamodb": "^3.0"

This is a backport of 5983c48 but for SilverStripe 3