how to remove shadow from brush group

We need to know what we are looking at in your screenshot.  Is that a single stamp of your problem brush preset, or is it multiple strokes?  To clarify that use the mouse and click once to stamp the preset on a new layer. Copy the layer and set it to Multiply, and then make a few more copies.  Then upload a screenshot of what you see.

Does the preset consist only of dots, or does it have strokes.  The hair and skin brushes I used for many years is a set by Castrochew. You can still get the set from Deviant Art   Even if don't use them, you can get some useful tips from seeing how they work.

One of his presets has multiple parallel lines, and it works by having Angle Jitter set to Direction so that the lines stay in line with the brush stroke.  So if your preset is not just dots, try doing that. Set Anglee Jitter to Direction.

So if you can show us another screen shot of a single stamp of the preset.   I'll leave it to you to decide if copying the layer brings up the shadow, so long as you make it clear what we are looking at.

Then try setting angle jitter to Direction and see what that does. 

One last thought, but again it is only a guess until we see the single stamp of the brush, is if the preset you sampled has converging lines like below — which is similar to your screen shot but without the shadow — then we can predict that this will definitely produce a background shadow because the lines will smear.

This is the above with no settings applied, and with a 1% spacing

Well here's kama for you.  I was going to demonstrate the above preset with some settings, but my Wacom tablet has just died on me. I restarted Photoshop and Lazy Nezumi tells me the tablet drivers are toast, so I have reinstall them which means restarting the computer so I have to post this now.

Perhaps you have captured shadow in your brush preset? Can you show us brush tip without any additional setting applied? 

Make your dots more opaque not so soft. You do not need fade, but sometimes I add a little brightness jitter for color variability also.