Unfortunately unrooting isn't that simple a process. In fact, it's about as simple as installing a custom recovery & ROM.
Besides that, re-rooting, depending on what updates your device receives, can be even more difficult, if not impossible, as future updates occur (impossible isn't likely, though).
Here's the easiest thing you can do, which will unroot you and remove your custom recovery, but leave your NAND unlocked for future "possibilities".
First, make sure you've installed
HTC Sync.
Then, download this file:
Dev-Host.org - The Ultimate Free File Hosting / File Sharing Service
That's the latest stock OS (ROM) for the EVO 4G. It's a ROM Update Utility (RUU) that runs on your computer, and it will update your device to Android 2.3.5.
You will need to enable USB Debugging in Android for it to work (Settings -> Applications -> Development, check USB Debugging). Once you've installed HTC Sync, and enabled USB Debugging, when you connect your device to the computer, it should detect it and install the necessary drivers. Then you can run the RUU to update your device. This will also perform a factory reset, which will remove any data.