I want to remove the iOS 13 beta but I don’t have a computer

You cannot remove the Beta without a computer.

Removing the profile simply stops you from getting further Beta updates but does in any way remove the beta version already installed.

You need iTunes on a computer to wipe the device clean, and install a non-beta version of iOS.

See here: https://beta.apple.com/sp/betaprogram/unenroll

If you don't have a computer, you can borrow one from your friends, then follow these instructions:

How to uninstall iOS beta software - Apple Support

or

ask for help at an Apple Store or Authorized Service provider: Apple – Location Selector

Regards

Giulio