Need `removeProvider` function

[wbobeirne]

Description of the Issue

If there's some UI event that removes a provider as an option (e.g. removing a custom node) we'll need to remove it from the provider list as well. This is especially a problem if you add a node, enter something wrong, remove it, and re-add it. Shepherd will fail due to duplicate provider.

Steps to Reproduce

1. Add a custom node on mycrypto
2. Remove the custom node
3. Add it again

Error: Provider config {your URL} already exists