Remove nb.metadata.file_extension fixes when upstream PR merged

[chrisjsewell]

See jupyter/jupyter-sphinx#127

Then the upcoming changes in #175 and

MyST-NB/myst_nb/cache.py

Lines 185 to 191 in afba1fe

	# This is a 'fix' for jupyter_sphinx, which requires this value for dumping the
	# script file, to stop it from raising an exception if not found:
	# Normally it would be added from the executed notebook but,
	# since we are already logging an error, we don't want to block the whole build.
	# So here we just add a dummy .txt extension
	if "language_info" not in ntbk.metadata:
	ntbk.metadata["language_info"] = nbf.from_dict({"file_extension": ".txt"})

can be removed

[choldgraf]

ah nice - if you get that patch in to jupyter-sphinx, I'll cut another patch release and then we can close the myst-nb PR I've got, I agree this is a better fix