don't need openssl

Signed-off-by: William Casarin <jb55@jb55.com>

shell.nix

@@ -1,5 +1,5 @@
 { pkgs ? import <nixpkgs> {} }:
 with pkgs;
 mkShell {
-  buildInputs = [ secp256k1 openssl gdb ];
+  buildInputs = [ secp256k1 gdb ];
 }