remove custom no-upnp bitcoind builds

Disabling upnp via compilation brings no substantial security benefits.
There's no way to inadvertently enable upnp, it must be set explicitly
via bitcoind.extraConfig.

But it's a huge hassle for new users who have to recompile bitcoind
before being able to use nix-bitcoin.

Also, elementsd is currently built with upnp support by default.

pkgs/pinned.nix

@@ -4,9 +4,9 @@ let
   nixBitcoinPkgsUnstable = import ./. { pkgs = unstable; };
 in
 {
-  bitcoin = unstable.bitcoin.override { miniupnpc = null; };
-  bitcoind = unstable.bitcoind.override { miniupnpc = null; };
   inherit (unstable)
+    bitcoin
+    bitcoind
     clightning
     lnd;
   inherit (nixBitcoinPkgsUnstable) electrs;