Remove support for `libsodium` fork

As per discussion in #615 it seems there is no longer a point in supporting a fork of libsodium with custom VRF implementation in it. In order to reduce complexity and avoid maintenance overhead for keeping a fork of libsodium and supporting different modes of compilation in `cardano-crypto-praos` we need to remove `external-libsodium-vrf` cabal flag and make it the default.

After this ticket is resolved we will always rely on the `cbits` wired in with `cardano-crtypto-praos`, at which point we could deprecate the [fork of libsodium](https://github.com/IntersectMBO/libsodium) maintained specifically for Cardano