Remove obsolete NPM certificate repository

dns_sync/config.py

@@ -107,13 +107,6 @@ class AppConfig:
     domain: str | None
     include_disabled: bool = False
 
-    @property
-    def proxy_url(self) -> str:
-        return (
-            f"{self.proxy_api_scheme}://"
-            f"{self.npm_endpoints[0].address}"
-        )
-
     @classmethod
     def from_env(cls, include_disabled: bool = False) -> "AppConfig":
         try: