You might be able to do something with either the levels tool or threshold
Photoshop is magical in many ways, but there are limitations. To remove the writing on the reverse side, you would need to select it somehow. Photoshop let's you select by area or by contrast. Neither of these would work well on your image because there is very little contrast between what you want to remove and what you want to keep, and the area you want to remove is not isolated, it is throughout the whole image. That is why you've not seen a solution posted.
If the ink on the reverse side of the paper is not bleeding through to the other side, you could try putting a dark paper, black or dark brown, on top of the sheet that you are scanning. That will prevent light from coming through the top of the paper.
If the ink is bleeding through to the other side, the only thing that might hellp you is that the bleed through is somewhat lighter than the writing you want to keep. You could try this:
Add a new layer underneath the image and fill with the paper color. Since the paper is not uniform color throughout, you could select a lighter and darker paper color as your foreground/background colors and run Filter > Render > Clouds to give the layer a little variation.
Back on the original layer, make a selection of the color you want to remove. Below I made a selection and tried not to include the darker brown that you want to keep.
Then I chose Select > Similar, and Select > Inverse, and clicked the layer mask icon at the bottom of the layers panel.
I ended up with this:
Next step would be to paint with black in the mask to hide any residual writing from the reverse side.
Would you use the levels tool or threshold on the image of the obverse side of the paper or on the image of the reverse side of the paper, or both images?