How to remove HDR effect on photos on iPhone 13?

Salutations m28i, 

We’d like to direct you to the following article from the iPhone User Guide for iOS 15 - Adjust HDR camera settings on iPhone

Based on this article, managing HDR can be done on some iPhone models, though iPhone 13 does not seem to be one of them. You can double verify that - 

Turn off automatic HDR

By default, iPhone automatically uses HDR when it’s most effective. On some iPhone models, you can manually control HDR instead.

• On iPhone XS, iPhone XR, iPhone 11 models, iPhone SE (2nd generation), and iPhone 12 models: Go to Settings > Camera, then turn off Smart HDR. Then from the camera screen, tap HDR to turn it off or on.

• On iPhone 8, iPhone 8 Plus, and iPhone X: Go to Settings > Camera, then turn off Auto HDR.

To turn HDR back on from the camera screen, tap HDR, then tap On.

• On iPhone 7, iPhone 7 Plus, and earlier: Tap HDR at the top of the camera screen, then tap Off.

You can disable HDR for video though - 

Turn HDR video off and on

On iPhone 12 models and iPhone 13 models, iPhone records video in Dolby Vision HDR for true-to-life color and contrast. To turn off HDR video recording, go to Settings > Camera > Record Video, then turn off HDR Video.

Thanks.