How should I go about removing a ceiling mounted downrod light?
If you want to remove the entire fixture from the ceiling then you do not need to undo the wiring below that you have shown in the photo. 

You only need to disconnect the wires in the ceiling box  that the fixture is attached to. 

You do not need and electrician to do this, ( but you may feel more comfortable hiring one or a handyman if you are not knowledgeable about electricity ) you do need to turn off the breaker that supplies power to the light and a safe ladder to get up there and an assistant. 

Inside the box the wires from the fixture should be connected to the wires supplying power to the box, they should be connected with wire nuts .  ( There may be other wires in the box as well, connected or not connected to the fixture wires ) 

Because the fixture is so big and heavy you will need an assistant to hold/support the fixture while you work on unscrewing the wire nuts that connect the wires from the fixture to the house wiring. Once you have disconnected the fixtures wires ( Black Hot, White neutral and Green/bare copper ground ) your assistant can take away the fixture and then  you will need to put the wire nuts back on the wires in the box. You should have a non contact voltage tester to verify that there is no power to any of the wires in the box before you proceed. 

If there are more then one set of wires that were connected with the wire nuts do not undo them just twist the wire nuts back onto the wires. 

This is a generalization because i can not see inside you ceiling box. 
You should watch some YouTube videos on how to change a light fixture so you have a better understanding of the processes. 

EDIT: Before you do anything in the box Take a photo and/or take notes on how things are connected in the box so if things go wonky in the future or you have to come back to it at a later date you have a reference on how it is and should be wired.