Remove cache tag rule for plugin managers

[Berdir]

Saw this in the entity_embed project.

  Line   src/EntityEmbedDisplay/EntityEmbedDisplayManager.php                  
 ------ ---------------------------------------------------------------------- 
  33     config:entity_view_mode_list cache tag might be unclear and does not  
         contain the cache key in it.       

This is very opinionated and IMHO not correct.

Cache tags in general in plugin managers are somewhat discouraged for performance reasons and explicit invalidation is recommended. but there's no requirement for them to include the cache key or be "clear".

Cache tags in plugin managers and other frequently used things should only be used if there a unknown amount of cache variations.

[mglaman]

Finally getting to this. I agree, it's very old code and an idea I once thought was a good one.