Remove deprecation notice for --mainnet flag

[nkuba]

We planned to remove the --mainnet flag, as it seemed unnecessary, as the client running without any network flag runs for mainnet by default. But from an infrastructure administrator perspective, the flag may be actually useful when running the application on multiple environments. It's much easier to maintain a common template where the administrator will have to change the value of the flag rather than set the flag for some environments and remove it for mainnet.

When running the application for mainnet an administrator will have two possibilities:

• add --mainnet flag,
• don't define any network flag.

When running the application for testnet an administrator will have to add --testnet flag.

[dimpar]

Left just one minor comment. Looks good to me.