Steps tp reproduce:
Create a product.
Create a yum repository with URL, sync and everything you'd do normally.
Create another yum repo without a URL.

Create a simplified ACS and add the product to it.
Try removing that product from the ACS. It fails.

<pre>
undefined method `smart_proxy_id' for nil:NilClass (NoMethodError)
/usr/share/gems/gems/katello-4.7.0.23/app/lib/actions/pulp3/alternate_content_source/delete.rb:6:in `plan'
</pre>

We probably need to test presence of smart_proxy_acs before planning Pulp3::Orchestration::AlternateContentSource::Delete here: https://github.com/Katello/katello/blob/449e319762f962c33dafd8b123f3f7f1d9feb1f5/app/lib/actions/katello/alternate_content_source/update.rb#L74