fix: remove legacy overrides

Author: heitorlessa

docs/overrides/assets/images/powertools_docs_thumbnail.png

@@ -56,7 +56,6 @@ theme:
     repo: fontawesome/brands/github
   logo: media/aws-logo-light.svg
   favicon: media/aws-logo-light.svg
-  custom_dir: docs/overrides
 
 markdown_extensions:
   - admonition