docs: Remove some outdated references to thumbnailing.md doc.

The doc was removed in 405bc8dabf

zerver/lib/markdown/__init__.py

@@ -662,8 +662,6 @@ class InlineInterestingLinkProcessor(markdown.treeprocessors.Treeprocessor):
             and (not already_thumbnailed)
             and user_uploads_or_external(image_url)
         ):
-            # See docs/thumbnailing.md for some high-level documentation.
-            #
             # We strip leading '/' from relative URLs here to ensure
             # consistency in what gets passed to /thumbnail
             image_url = image_url.lstrip("/")

zerver/lib/thumbnail.py

@@ -1,4 +1,3 @@
-# See https://zulip.readthedocs.io/en/latest/subsystems/thumbnailing.html
 import os
 import sys
 from urllib.parse import urljoin

zerver/views/thumbnail.py

@@ -1,4 +1,3 @@
-# See https://zulip.readthedocs.io/en/latest/subsystems/thumbnailing.html
 from typing import Optional, Union
 
 from django.contrib.auth.models import AnonymousUser