DOC: Remove unused custom_template config

This configuration was recently updated to check if the target
exists [1]. As we do not acutally use custom plugins, removing
the setting fixes CI.

[1]: mkdocstrings/mkdocstrings@370a61d

mkdocs.yml

@@ -47,7 +47,6 @@ plugins:
 
   - mkdocstrings:
       enabled: false
-      custom_templates: templates
       default_handler: python
       handlers:
         python: