chore(librarian): remove obsolete action to update the root changelog

[parthea]

Updating the root changelog is handled by the following code. The github workflow updatechangelog.yml is no longer needed.

google-cloud-python/.generator/cli.py

Lines 282 to 286 in 2b3912f

	_update_global_changelog(
	f"{repo}/CHANGELOG.md",
	f"{output}/CHANGELOG.md",
	[new_library_config],
	)

google-cloud-python/.generator/cli.py

Lines 1444 to 1450 in 2b3912f

	if is_mono_repo:
	# only a mono repo has a global changelog
	_update_global_changelog(
	f"{repo}/CHANGELOG.md",
	f"{output}/CHANGELOG.md",
	libraries_to_prep_for_release,
	)