It doesn't say that the save is negated, so it remains. You still deal maximum damage, as it says, just as if you had rolled for it. After dealing that damage, they may save to mitigate the damage, as usual when such damage is dealt.
Overchannel does only what it says: it changes how much damage the spell deals. When a spell that offers a saving throw deals damage, you figure out the damage first and then the saving throw second, and it works like that regardless of Overchannel's effect. Overchannel just changes how you figure the damage, nothing after.