Expire support for root_doc "contents" rewriting

[timhoffm]

Since 2.0, there has been special code to support a root doc "contents", which has not been specified in the config:

sphinx/sphinx/config.py

Lines 769 to 781 in 116a430

	def check_root_doc(app: Sphinx, env: BuildEnvironment, added: Set[str],
	changed: Set[str], removed: Set[str]) -> Iterable[str]:
	"""Adjust root_doc to 'contents' to support an old project which does not have
	any root_doc setting.
	"""
	if (app.config.root_doc == 'index' and
	'index' not in app.project.docnames and
	'contents' in app.project.docnames):
	logger.warning(__('Since v2.0, Sphinx uses "index" as root_doc by default. '
	'Please add "root_doc = \'contents\'" to your conf.py.'))
	app.config.root_doc = "contents"
	return changed

As cleanup / maintanance, I'd like to remove this code. Question: Is the log warning sufficient so that we can immediately remove, or do you want a formal deprecation?