Remove Futurenet from network selector

[aristidesstaffieri]

What problem does your feature solve?

Futurenet is no longer a priority for Freighter, let's remove it from the network selector.
Users who rely on Futurenet can configure it as a custom network using their own RPC.

What would you like to see?

Remove Futurenet from the network selector, and clean up any code related to handling Futurenet differences.

What alternatives are there?

N/A

[JakeUrban]

No need to do this in the short-term, but long-term we will remove support for using multiple networks in Freighter. We can add this ticket to a milestone for this broader work.