Layout Builder defaults should work regardless of the ability to provide overrides

Problem/Motivation

Layout Builder overrides require that the entity has a canonical route available, which is usually tied to the "default" view mode.

Layout Builder defaults have no such requirement, as they are usable by any view mode.

layout_builder_entity_type_alter() provides a layout-builder link template only for entity types that:

• are fieldable
• have a view builder
• have a canonical link template

This logic is then used throughout the module to determine which entities get the Layout Builder UI provided.

However, the logic should instead be split, and clarified.
Overrides should only work for entity types that:

• are fieldable
• have a view builder
• have a canonical link template

Defaults should work for entity types that:

• are fieldable
• have a view builder
• have a Field UI base route

Proposed resolution

Instead of having this logic within this hook and referencing it from each plugin, keep the logic stored within each plugin.
The logic will continue to need to be copied into LayoutBuilderLocalTaskDeriver, as that still remains a workaround for #2936655: Layout Builder should use the toolbar modes system once it exists.

Remaining tasks

N/A

User interface changes

N/A

API changes

The "layout-builder" link template is removed.

Data model changes

N/A