README.md: Remove reference to OpenSSL
README.md: Remove reference to OpenSSL

The OpenSSL files were removed in a6ca68a70c3bc0b2e6abad70346c99642a896f9f.

https://salsa.debian.org/debian/nix/issues/3

rycee/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,pikajude/nix,rycee/nix,ehmry/nix,ehmry/nix,ehmry/nix,NixOS/nix,ehmry/nix,rycee/nix,ehmry/nix,pikajude/nix,NixOS/nix,NixOS/nix,ehmry/nix,pikajude/nix