markdown: Delete unused log_markdown_error.

Its used was removed in 50ed378dd6.

zerver/lib/markdown/__init__.py

@@ -2102,13 +2102,6 @@ _privacy_re = re.compile('\\w', flags=re.UNICODE)
 def privacy_clean_markdown(content: str) -> str:
     return repr(_privacy_re.sub('x', content))
 
-def log_markdown_error(msg: str) -> None:
-    """We use this unusual logging approach to log the markdown error, in
-    order to prevent AdminNotifyHandler from sending the sanitized
-    original markdown formatting into another Zulip message, which
-    could cause an infinite exception loop."""
-    markdown_logger.error(msg)
-
 def get_email_info(realm_id: int, emails: Set[str]) -> Dict[str, FullNameInfo]:
     if not emails:
         return dict()