Remove references to ConfigurablePluginInterface

Problem/Motivation

In #2946122: Deprecate ConfigurablePluginInterface and replace with an interface that doesn't extend DependentPluginInterface → We deprecated ConfigurablePluginInterface in favor of a new ConfigurableInterface for Plugins that does not extend DependentPluginInterface. For the sake of backwards compatibility, plugins in core continued to implement both interfaces and interface checks in core continue to check for both interfaces.

Proposed resolution

Prior to Drupal 9.0.0, References to ConfigurablePluginInterface should be removed from core. Alternatively we may decide to immediately deprecate references to ConfigurableInterface and restore ConfigurablePluginInterface, simply without extending DependentPlugininterface.
Either way, one of the interfaces should be removed.

Remaining tasks

Decide which name to keep in Drupal 9.0.0, and remove references to the other one.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

References to ConfigurablePluginInterface have been removed.