The power to approve the removal a state is delegated, as per Article IV. It specifically mentions one way, by implication any removal will need Congressional approval.
The Constitution states, 'No Title of Nobility shall be granted by the United States'.
Read Article I, Section 10, Clause 1 again.
Ok, lets play the implication game. The states aceded to the Constitution unilaterally, so by implication, they can leave unilaterally.
Read Article I, Section 10, Clause 1 again.
Article I § 10 clause 1: 'Section 10 No State shall ... grant any Title of Nobility.' The election of a President is not a conferring of a title of nobility by a state.
Read Article I § 9 clause 8 again: 'No Title of Nobility shall be granted by the United States.'