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I bought a complete lower for an AR pistol build. Do I have to replace the carbine buffer tube with a pistol tube or can I just remove the stock and call it good?

So long as the lower was transferred to you as an "other" and not a "rifle" on the 4473 you should be good to remove the stock and buffer tube and replace them with a pistol buffer tube. Most FFLs should be wise to this, but that's not always the case. If you can check with your FFL to see what it was transferred as you'll know for sure. The reason for this is the ATF allows conversions as referenced below:

Other -> Rifle
Other -> Pistol
Pistol -> Rifle
Rifle -> Pistol

It's silly, but it's the law for now.

As for pulling the stock off, it's fairly commonly held that you can pull the stock off and build a pistol, but it still leaves some grey areas. If for instance you own no other ARs but have a stock sitting around with a buffer tube that will accept a stock on the gun there's always the chance you could get hit with the charge of "constructive intent". The chances are slim as I can find no reference to the ATF ever charging anyone with this as a stand alone crime (it's usually an add-on to more common/serious charges); still, the possibility exists. If however you own other ARs it'd be hard to prove as you can always argue it's a spare for one of your rifles. At the same time a pistol buffer can be had for less than $20, so it's cheap insurance to just change out to one of these.

I'd recommend getting something like a CAA saddle. They fit onto a regular carbine tube, so a stock can't be installed, and they essentially feel like the top half of a Sopmod stock (comfortable). They're not pricey either. I use them exclusively for my pistols now instead of braces.