Components stopped tracking the `category` in their `Component` config entity

What?

A number of ComponentSource plugins (Blocks and Single-Directory Components) provide a `category` and we were tracking that in their respective `Component` config entities.

Why?

After we added a "Manage Library" tab and the ability to classify them in folders, that is not needed anymore.

Component sources will provide the initial folder when they are discovered for the first time (\Drupal\canvas\ComponentSource\ComponentSourceInterface::determineDefaultFolder was added), but editors can re-organize them.