As they're essentially instructions that point MetaMask in the correct direction (towards an RPC endpoint), all the networks you add are removable. Follow the steps below for the platform you're using:
Click the network selector at the top of the app to bring up the list of networks you've previously added. Find the network you want to remove and click the cross that appears when you mouse over it.
Click 'Delete' on the menu that appears.
Tap the hamburger icon in the top left of the app and tap 'Settings'.
Find 'Networks' in the settings menu.
You'll now see a list of the networks you've previously added to your MetaMask:
Long press on the network you want to remove to bring up a prompt confirming your intention, and then tap 'Remove'.
If you need to re-add a network, just follow the same steps you used to add it originally: via Chainlist or manually.
Please note that testnets will be displayed automatically and cannot be removed. If you don't want to see them, there's a toggle in settings that allows you to hide them. See here for instructions.