fix: Remove unused global declaration

Remove the global declaration for 'has_shown_yaml_error_message'.
This variable was deleted in a previous change, but the declaration
was left behind by mistake. This fixes flake8 error F824.

Author: jstvz

cumulusci/utils/yaml/cumulusci_yml.py

@@ -280,7 +280,6 @@ class ErrorDict(TypedDict):
 
 def _log_yaml_errors(logger, errors: List[ErrorDict]):
     "Format and log a Pydantic-style error dictionary"
-    global has_shown_yaml_error_message
     plural = "" if len(errors) <= 1 else "s"
     logger.warning(f"CumulusCI Configuration Warning{plural}:")
     for error in errors: