Clean up configuration

Hi, thanks for your work on the module! I had a suggestion.

Problem/Motivation

At the moment AFAICT the configuration in prevnext.settings:prevnext_enabled_entity_types is redundant - it can be inferred from prevnext_enabled_entity_bundles.

Proposed resolution

Only store and check prevnext.settings:prevnext_enabled_entity_bundles.

Remaining tasks

Do it!

User interface changes

It doesn't require a change in UI.