I don't like using an angle grinder for deep cuts. Pinch the cutting wheel, and the wheel shatters. If you go this route, face shield, dust mask, gloves and long sleeves.
Most concrete saws I've seen run wet. If you do this, then use a GFCI circuit for your tools (and test the GFCI...) Wet means a bigger local mess, but you don't have concrete dust throughout your house.
I would see if I can find a cutting wheel that would fit a circular saw. This allows you to get within 1.25" of the edge, with some larger uncut bits in the corner. Do the back edge first. Then support the front edge. Do the right edge next. It's awkward as you need to come OUT with the saw (unless it has a left handed base plate) Given the thickness of your shelf, you may have to do this from the bottom too. If so, do the bottom first. a 300 pound slab o concrete falling on your head can ruin your day.
Might ask at a rental tool shop if they ahve something that does this sort of magic.
If they make concrete blades for reciprocating saws, you may be able to complete more of the cuts, but I think you will still end up about an inch from the wall.
Now a few whacks with a sledge should bring it down.
At this point you have a 1.25 inch stub and a ragged corner. You can see where the reinforcing runs now, so set the depth to 1.4 inches and cut into the face you just made above and below the reinforcing. This gives you a narrow ledge with no reinforcing with the base of the cut a bit deeper into the wall. If you can, extend this cut into the corner.
using a stone cutter's chisel (brick chisel) whack the corner where the shelf meets the wall. I'd try doing several passes, along the full length of shelf, the idea that you want a crack started that will break cleanly. Eventually it should crack off. With the slightly deeper cut, the break should be under the wall plane, so that you can patch rather then grinding down a rough break.
For the layer with the reinforcing, your angle grinder may shine. This is likely a fairly thin layer to blade pinching is less likely.
The bottom layer will be a very awkward swing for hammer and chisel. Perhaps a 1/8" groove cut with the angle grinder, then whack from the top.
Note: I am not a mason or a concrete worker. Advice worth what you have paid for it.