Replace _get_op_circuit with op.untagged (#3893)
Replace _get_op_circuit with op.untagged (#3893)

Since `Operation` provides a canonical way to get the untagged version of itself, we should use it instead of writing a new method to do the same thing.

Using `op.untagged` in #3634 removes any residual blocking effects from #3678, although I'm still in favor of finding a proper resolution to #3678.
quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,balopat/Cirq,quantumlib/Cirq,balopat/Cirq,balopat/Cirq