Welcome to the forum
The problem is that to unroot needs a RUU (ROM Update Utility), and these are Windows executables. There's a way of extracting the core of the RUU as a zip which can be flashed through the bootloader. Flashing that way doesn't require a PC to do, but you need one to extract the ROM zip from the RUU. So one way or another Windows is involved.
Did you save a nandroid of your stock ROM before rooting (i.e. after installing the custom recovery but before modifying the ROM)? If so restoring that will give you an unrooted ROM, and then we'd just need to find a stock recovery you could flash via fastboot, then relock the bootloader and you're done.
Edit: OK, I may have an answer. Note that this will wipe the phone completely, so back up anything of importance first.
Crushalot has a couple of resources which should work without needing a PC. One can be found on his website, where he's provided a set of "ruu mode zips"
here, which includes at least one ATT file (with JB 4.3, so more recent than your current software). If you then follow the "return to stock guide" link in the top bar of that site and look for "restoring using a RUU.zip" that will explain how to use it. This is, as the name implies, another way of running a RUU, so you need to use an ATT package.
Scotty85 has a post describing the same method (for ATT)
here, which may be easier for you since the post describes just this one method whereas crushalot's guide describes many options.
Crushalot also posted an alternative version
here. This seems to be just a stock ROM and recovery in a zip which can be flashed from the custom recovery the same way as a custom ROM. That won't update radio, firmware or hboot, which means the ones you have may not be the versions which correspond to the ROM this would install. If you were thinking of staying stock and accepting future over the air updates the ruu zip approach might be better for that reason, but this one is perhaps simpler (I've never used this, but have used the fastboot zip method to update the firmware of my phone many times).
Note that if you are S-Off you can restore the text at the top of the hboot screen to "locked" rather than "relocked" as well.
