Remove stale module-level docstrings pointing to docs

These three module docstrings only restated the module's name and
pointed to the corresponding docs/topics/*.rst page, which already
documents the module in full. No unique information was lost.

Partial pass on #7699; a broader sweep is still needed.

scrapy/downloadermiddlewares/downloadtimeout.py

@@ -1,9 +1,3 @@
-"""
-Download timeout middleware
-
-See documentation in docs/topics/downloader-middleware.rst
-"""
-
 from __future__ import annotations
 
 from typing import TYPE_CHECKING

scrapy/extension.py

@@ -1,9 +1,3 @@
-"""
-The Extension Manager
-
-See documentation in docs/topics/extensions.rst
-"""
-
 from __future__ import annotations
 
 from typing import TYPE_CHECKING, Any

scrapy/loader/__init__.py

@@ -1,9 +1,3 @@
-"""
-Item Loader
-
-See documentation in docs/topics/loaders.rst
-"""
-
 from __future__ import annotations
 
 from typing import TYPE_CHECKING, Any