See #6158 / #6158 (comment)

Closes #4800

The plugin is broken, unmaintained and hasn't been updated in years:
https://github.com/streamlink/streamlink/commits/6.10.0/src/streamlink/plugins/facebook.py

$ ./script/test-plugin-urls.py facebook
:: https://www.facebook.com/119555411802156/posts/500665313691162/
!! No streams found
:: https://www.facebook.com/SporTurkTR/videos/798553173631138/
!! No streams found
:: https://www.facebook.com/nos/videos/1725546430794241/
!! No streams found
:: https://www.facebook.com/nytfood/videos/1485091228202006/
!! No streams found
:: https://www.facebookwkhpilnemxj7asaniu7vnjjbiltxjqhye3mhbshg7kx5tfyd.onion/SporTurkTR/videos/798553173631138/
!! Error while fetching streams

The current implementation apparently aimed at supporting videos from user feeds. As said in #6158, I don't have a Facebook account. Creating a new one without valid personal data doesn't work reliably and they'll block/ban you immediately, which makes this unnecessarily difficult. I therefore don't have any interest in fixing this.

Facebook's "new" livestreaming service however was never supported by the plugin
https://www.facebook.com/watch/live
If anyone wants support for this, they can submit a new working plugin implementation. This plugin removal simply gets rid of the broken code, so that we don't incorrectly advertise support for that site.