Remove redundant SILType::isMoveOnlyNominalType (NFC)

and implement `SILType::isPureMoveOnly` in terms of `Type::isPureMoveOnly`.

lib/SIL/Verifier/SILVerifier.cpp

@@ -6016,7 +6016,7 @@ public:
     auto type = ddi->getType();
     require(type == ddi->getOperand()->getType(),
             "Result and operand must have the same type.");
-    require(type.isMoveOnlyNominalType(),
+    require(type.isPureMoveOnly(),
             "drop_deinit only allowed for move-only types");
     require(type.getNominalOrBoundGenericNominal()
             ->getValueTypeDestructor(), "drop_deinit only allowed for "