Fix Delay instruction's new validate is_real check

The definition of the Delay's validate_parameter method was changed in
PR #6225, this just updates it to use the new
ParameterExpression.is_real() so it works in a world with symengine.

qiskit/circuit/delay.py

@@ -77,7 +77,7 @@ def validate_parameter(self, parameter):
         elif isinstance(parameter, ParameterExpression):
             if len(parameter.parameters) > 0:
                 return parameter  # expression has free parameters, we cannot validate it
-            if not parameter._symbol_expr.is_real:
+            if not parameter.is_real():
                 raise CircuitError(f"Bound parameter expression is complex in delay {self.name}")
             fval = float(parameter)
             if self.unit == "dt":