Remove Slack notifications
Remove Slack notifications

They're too much noise since notifications include PRs by default.
We'll use notification on a combined build in installer instead.

silverstripe/silverstripe-asset-admin,silverstripe/silverstripe-asset-admin,open-sausages/silverstripe-asset-admin,open-sausages/silverstripe-asset-admin,open-sausages/silverstripe-assets-gallery,open-sausages/silverstripe-assets-gallery