Clean up and document decorators replaced by static maps

[ybnd]

Describe the bug In #2750 some entry component decorators were replaced with static maps, e.g. rendersMenuItemForType and rendersBrowseBy.

There are still a few problems that should be addressed:

• The original decorators still exist, but aren't used. If they aren't meant to be used, they should be removed from the codebase to avoid confusion.
  • Specifically rendersBrowseBy looks broken, as it uses the new static map and the old dynamic map constants interchangeably
• It's unclear how theme support should work for browse-by components now. This should be clarified in the code.
  • Right now, the custom theme versions of these components are not included in the mapping, so they will never be visible in the UI
• The static map pattern replacing rendersBrowseBy is very difficult to read