I am with Jeff and mchockey... It is certainly not there and I have looked ALL over. VERY VERY odd. Worse for me because the incoming notification disrupts my phone call (makes the audio very static-y, both incoming and outgoing preventing hearing clearly) when on bluetooth.
I have traced the problem to the notification arriving while on a call by using Verizon's support number (*611) and having it send me my current billing status (option 1, then 1) via text while I am on bluetooth: should anyone like to test it. The static lasts exactly 20 seconds (default notification length). I have tried turning of notifications in my messaging app(s) so I would get no notifications at all, but it still happens.
So I am trying very hard to find where I can stop the IN-CALL notifications. And as Jeff and mcHockey note, the typical settings are just not there (but are on all my other phones: Samsung S3, S4, HTC Evo, etc.). I am using a S-Off'd, rooted, but stock VZ HTC One M8 and it does not have the setting anywhere that I can find, and I have spent hours....