Deep subsumption: unify mults without tcEqMult

Ticket: #26332 (closed)

As seen in #26332 (closed), we may well end up with a non-reflexive multiplicity coercion when doing deep subsumption. We should do the same thing that we do without deep subsumption: unify the multiplicities normally, without requiring that the coercion is reflexive (which is what tcEqMult was doing).