Big Bang Bot: Update CODEOWNERS

Action	User	Reason
Remove	@mark.nelson	Username is not present in either team or contributor whitelist.
---------	---------	-----------
Error	@	Too few codeowners, minimum is: 3

This package is not currently tracked in the team charter mission team manifest. As such, only expired/unrecognized users have been removed. If you would like this package to sync with the manifest file, please refer to updating team charter manifest.