Remove block.module dependency from block_content

Problem/Motivation

1. The block_content module is a block factory. It creates blocks plugins by deriving over reusable Block Content entities (\Drupal\block_content\Entity\BlockContent).
2. The block module mainly offers block placement functionality.

Normally, block_content.module should not need block.module in order to function properly. The blocks produced by block_content can be placed on page by Layout Builder, Display Suite, Panels and even... (core) Block module.

But there are some dependencies on block.module exposed in block_content.module:

• The administer blocks permission.
• The block.admin_display route.
• Unnecessarily installing block in tests
• Redirects for insert/update with BlockContent entity form
• Delete form and delete hook
• ...anything else?

But, I think the above, dependencies were just a compromise, inherited from earlier Drupal versions. I see them only polluting the intra-module relationships by an unneeded dependency.

Proposed resolution

Now that we have decoupled block_content from the block library, I believe we can remove dependency

To be discussed...

• Move block.admin_display route in core? - Believe this is no longer needed
• What about the shared permission? - Believe this is no longer needed

Remaining tasks

• Review outline in #3206952-22: Remove block.module dependency from block_content
• Review proposed MR

User interface changes

None

API changes

Move appropriate customisations to Block or Block Content modules.
Add accommodations to testing in Block Content that relies on block placement to avoid block.module

Data model changes

None

Release notes snippet

@todo