Remove disused 'markdown_library' value from markdown.settings

Problem/Motivation

When upgrading from 8.x-1.x to 8.x-2.x, the schema of markdown.settings changed. See #3142418: Support multiple libraries per plugin → .

8.x-1.x:

markdown.settings:
  type: config_object
  label: 'Markdown settings'
  mapping:
    markdown_library:
      type: string
      label: 'Markdown library'

8.x-2.x:

# Markdown Settings.
markdown.settings:
  type: config_object
  label: 'Markdown Settings'
  mapping:
    default_parser:
      type: string

markdown_post_update_8950() sets the new default_parser value; however it fails to remove the removed markdown_parser value.

See https://git.drupalcode.org/project/markdown/-/blame/8.x-2.x/markdown.pos...

This results in a schema validation error:

array (
  'markdown.settings:markdown_library' => 'missing schema',
)

Steps to reproduce

1. Install Drupal 8 and the Markdown module version 8.x-1.x.
2. Upgrade to Markdown 8.x-2.x.
3. Observe validation error.
4. Upgrade to Drupal 10 and Markdown 3.0.x.
5. Observe validation error.

Proposed resolution

Add a post_update function to remove the markdown_libraryvalue, if it exists, from markdown.settings.

Remaining tasks

Open MR.

User interface changes

None.

API changes

None.

Data model changes

None. (Data model already changed.)