Can I remove / uninstall DD-WRT or 3rd-party firmware and revert back to the original firmware?

Yes, it is possible anytime.

If you have DD-WRT you can directly revert back to Linksys firmware using DD-WRT's web interface. If you are using HotspotWRT or CoovaAP, first you need to upgrade to OpenWRT, then DD-WRT to be able to revert it to Linksys Firmware. 

If you have DD-WRT on your router you have to do the following:

Download the proper firmware from Linksys (if you have a Linksys router) http://www.linksysbycisco.com/. Make sure you get the right firmware for your router.

Link for the firmware can be found on the bottom left, after you selected your router type and version.

Next, do a hard reset on the router (hold the reset button for 30 seconds, OR from the GUI, do a reset to factory defaults.

Next, flash the Linksys firmware in via the GUI taking care not to lose power, reset or otherwise disturb the router during flashing and for 10 MINUTES after it says it's complete....leave it alone.

After 10 minutes, do another hard reset. This time it's best to hold the reset button for 30 seconds.

After it boots, open a browswer and browse back to the GUI. This time you should see the Linksys firmware page, and your user name and password will have changed to: username = blank, password = admin or username=root, password=admin. You're done.