Yes. That's a bridge that allows the motor to run from AC and if you chop the supply voltage, the motor's inductive current will be pulled through the diode bridge, ie it will operate in slow-decay mode.
The motor will not be reversible with that bridge on there.
Edit: it's OK as long as you use the red and white wires provided, and don't directly connect to the motor terminals. Making such a direct connection and getting the polarity wrong will mean shorting your supply through the diodes.