He is no longer in office, so from what would be removing him?
Judgment in Cases of impeachment shall not extend further than to removal from Office, and disqualification to hold and enjoy any Office of honor, Trust or Profit under the United StatesU.S. Const., Art. I, Sec 3, Cl. 7.
That is correct. However,
the Party convicted shall nevertheless be liable and subject to Indictment, Trial, Judgment and Punishment, according to Law.Id.
It is conceivable he could be constitutionally subpoenaed and tried and convicted for his crimes in office.
Impeachment and removal are two different steps.
Clinton was impeached but not removed.