Simplify network configurations

[georgi-l95]

Proposal to remove previewnet, testnet, mainnet configuration for several reasons:

1. They are unused from most, if not all users.
2. It's almost as easier to start with whatever container you want, just by specifying using --network-tag or --mirror-tag or --relay-tag
3. Because of them, we need to overide env. varibles, which create unnecessery complications when used in CI.