I have been struggling with this RMM flag for more than 3 months. But finally I succeded to remove it completely from my phone and rooted it. And that thanks to Halabtech team.
NOTE : I'm sharing this method because it worked for me. It may not work for you. Use it at your own risk.
The steps are as follow :
1 - Download Combination file for your model, J600G v3 bootloader : J600GDXU3ARH5, J600F v3 bootloader : J600FXXU3ARH5
2 - Flash this combination file (AP slot in odin).
3 - Root yout phone with iRoot
4 - Download Halabtech Tool v0.5 and install it.
5 - Go to RMM Tab and check your phone model and hit start.
6 - RMM now has been removed. You can flash stock and twrp.