[PR #10058/12372d7 backport][3.12] Remove unreachable content length …

…check for chunked encoding (#10061)

aiohttp/web_response.py

@@ -181,14 +181,13 @@ def output_length(self) -> int:
 
     def enable_chunked_encoding(self, chunk_size: Optional[int] = None) -> None:
         """Enables automatic chunked transfer encoding."""
-        self._chunked = True
-
         if hdrs.CONTENT_LENGTH in self._headers:
             raise RuntimeError(
                 "You can't enable chunked encoding when a content length is set"
             )
         if chunk_size is not None:
             warnings.warn("Chunk size is deprecated #1615", DeprecationWarning)
+        self._chunked = True
 
     def enable_compression(
         self,
@@ -493,8 +492,6 @@ async def _prepare_headers(self) -> None:
             if not self._must_be_empty_body:
                 writer.enable_chunking()
                 headers[hdrs.TRANSFER_ENCODING] = "chunked"
-            if hdrs.CONTENT_LENGTH in headers:
-                del headers[hdrs.CONTENT_LENGTH]
         elif self._length_check:  # Disabled for WebSockets
             writer.length = self.content_length
             if writer.length is None: