Allow deletion of selected network

This is to fix the extension issue: https://github.com/MetaMask/metamask-extension/issues/34457

[The Global Network Selector is being removed](https://github.com/MetaMask/core/issues/5737) In the new version of extension, we can select muti active networks, so we can remove any of them including the current(selected) one.

No implementation file or test is named. Start by reading the linked extension issue #34457 and core issue #5737, then locate the network-selection and deletion entry point. Done means a selected network can be deleted when multiple active networks exist, including the currently selected one, with coverage for that behavior.

Written by the indexing model from the issue text.