Remove tabs-migration-extension

All versions brought in by the playbook now use asciidoctor tabs

antora-playbook.yml

@@ -6,10 +6,6 @@ antora:
   - require: '@springio/antora-extensions/inject-collector-cache-config-extension'
   - '@antora/collector-extension'
   - '@antora/atlas-extension'
-  - require: '@springio/antora-extensions/tabs-migration-extension'
-    # uncomment this option to save the migrated content to the worktree
-    #save_result: true
-    unwrap_example_block: always
   - require: '@springio/antora-extensions/publish-docsearch-config-extension'
     index_name: 'spring-security-docs'
     root_component_name: 'security'