[cleanup] Remove unused code (#8968)

Authored by: pukkandan, seproDev

README.md

@@ -2175,7 +2175,7 @@ Some of yt-dlp's default options are different from that of youtube-dl and youtu
 * yt-dlp versions between 2021.09.01 and 2023.01.02 applies `--match-filter` to nested playlists. This was an unintentional side-effect of [8f18ac](https://github.com/yt-dlp/yt-dlp/commit/8f18aca8717bb0dd49054555af8d386e5eda3a88) and is fixed in [d7b460](https://github.com/yt-dlp/yt-dlp/commit/d7b460d0e5fc710950582baed2e3fc616ed98a80). Use `--compat-options playlist-match-filter` to revert this
 * yt-dlp versions between 2021.11.10 and 2023.06.21 estimated `filesize_approx` values for fragmented/manifest formats. This was added for convenience in [f2fe69](https://github.com/yt-dlp/yt-dlp/commit/f2fe69c7b0d208bdb1f6292b4ae92bc1e1a7444a), but was reverted in [0dff8e](https://github.com/yt-dlp/yt-dlp/commit/0dff8e4d1e6e9fb938f4256ea9af7d81f42fd54f) due to the potentially extreme inaccuracy of the estimated values. Use `--compat-options manifest-filesize-approx` to keep extracting the estimated values
 * yt-dlp uses modern http client backends such as `requests`. Use `--compat-options prefer-legacy-http-handler` to prefer the legacy http handler (`urllib`) to be used for standard http requests.
-* The sub-modules `swfinterp` is removed.
+* The sub-modules `swfinterp`, `casefold` are removed.
 
 For ease of use, a few more compat options are available:
 

yt_dlp/casefold.py

@@ -1,5 +0,0 @@
-import warnings
-
-warnings.warn(DeprecationWarning(f'{__name__} is deprecated'))
-
-casefold = str.casefold

yt_dlp/dependencies/__init__.py

@@ -53,9 +53,7 @@ except ImportError:
 
 try:
     import websockets
-except (ImportError, SyntaxError):
-    # websockets 3.10 on Python 3.6 causes SyntaxError
-    # See https://github.com/yt-dlp/yt-dlp/issues/2633
+except ImportError:
     websockets = None
 
 try: