Cant delete or edit any test networks

In the networks section of metamask settings, the default networks (mainnet, ropsten, etc) cant be edited or removed, at all. Should be able to remove these networks esp if I want to use my own node for ethereum mainnet thats not localhost, without having two network instances of ethereum mainnet, and its annoying to have a bunch of test networks that dont really get used filling up the list, gets annoying with many custom networks added (bsc, matic, xdai, etc) and the list is becomes long and networks are hard to find

Start in the MetaMask settings' Networks section and trace how default and custom networks are listed and selected. Done means users can edit or remove default networks, including test networks, and can configure a non-localhost mainnet without a duplicate entry; verify the network list remains usable with several custom networks.

Written by the indexing model from the issue text.