mcweb/backend/sources/feed_url.py unused, code is in metadata-lib, so remove the file

[philbudne]

https://github.com/mediacloud/web-search/blob/main/mcweb/backend/sources/feed_url.py contains only the function normalize which also appears (as the only function)normalize_url` in https://github.com/mediacloud/metadata-lib/blob/main/mcmetadata/feeds.py

The files only differ in:

1. function name
2. white space
3. metadata-lib moved tests to mcmetadata/test/test_feeds.py

web-search does not use the local copy, (only reference is commented out, in mcweb/backend/sources/serializer.py, mcweb/backend/sources/{models,tasks}.py use mcmetadata version), so to avoid confusion and possible future use and/or modification, mcweb/backend/sources/feed_url.py should be removed