Since Pres. Trump is no longer the President of the United States, Article 1, Section 3's stipulation that the Chief Justice must preside at the trial of a POTUS is no longer applicable.

The presiding officer in impeachment trials for non-presidents (judges, mainly, but apparently now ex-presidents too) is "whoever the Senate picks," and "oldest member of the Senate from the majority party" is pretty standard.

Whether or not Article 1 Section 3's "Chief Justice presides" requirement applies to this impeachment or not has been somewhat in dispute for the past few days -- since he was President when impeached but is no longer at trial -- and apparently it's on the cusp of being settled. (My guess is that Roberts wants to wash his hands of the whole thing and said "please no.")