[ConstraintSystem] NFC: Remove obsolete getPotentialBindings declaration

It has been related with `inferBindingsFor` but declaration was
left in the header.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -4951,7 +4951,6 @@ class ConstraintSystem {
   Optional<ConstraintSystem::PotentialBinding>
   getPotentialBindingForRelationalConstraint(PotentialBindings &result,
                                              Constraint *constraint) const;
-  PotentialBindings getPotentialBindings(TypeVariableType *typeVar) const;
 
   /// Add a constraint to the constraint system.
   SolutionKind addConstraintImpl(ConstraintKind kind, Type first, Type second,