Revert "Add rubocop-md"

[koic]

This reverts commit a30ef79.

rubocop-md is a unique gem that doesn't possess any cops. It inspects Ruby code within Markdown files. However, as can be seen from the disabling of cops in rails/rails, which cops to disable is documented in the inspection target repository:

  Exclude:
+   - '**/CHANGELOG.md'
+   - '**/2_*_release_notes.md'
+   - '**/3_*_release_notes.md'
+   - '**/4_*_release_notes.md'
+   - '**/5_*_release_notes.md'
+   - '**/6_*_release_notes.md'

 Layout/IndentationConsistency:
   Enabled: true
   EnforcedStyle: indented_internal_methods
+  Exclude:
+    - '**/*.md'

 Style/FrozenStringLiteralComment:
   Enabled: true
   EnforcedStyle: always
   Exclude:
     - 'actionview/test/**/*.builder'
     - 'actionview/test/**/*.ruby'
     - 'actionpack/test/**/*.builder'
     - 'actionpack/test/**/*.ruby'
     - 'activestorage/db/migrate/**/*.rb'
     - 'activestorage/db/update_migrate/**/*.rb'
     - 'actionmailbox/db/migrate/**/*.rb'
     - 'actiontext/db/migrate/**/*.rb'
+    - '**/*.md'

This means that any false positives in documentation need to be addressed as they arise, leading one to question whether this is the behavior users expect.

This PR resolves the above-mentioned usability issues by disabling rubocop-md. If users wish to enable rubocop-md, they simply need to add it explicitly to their .rubocop.yml. In other words, the use of rubocop-md is a deliberate choice made by the user.

I believe this is a better configuration for rubocop-rails_config.

[toshimaru]

still considering this. Let me have a little more time.