Remove unused/untested headers copy() method

Last usage was removed in encode#804

Author: jcugat

httpx/_models.py

@@ -481,9 +481,6 @@ def update(self, headers: HeaderTypes = None) -> None:  # type: ignore
         for header in headers:
             self[header] = headers[header]
 
-    def copy(self) -> "Headers":
-        return Headers(self.items(), encoding=self.encoding)
-
     def __getitem__(self, key: str) -> str:
         """
         Return a single header value.