remove unnecessary filtering code

lib/privatekey.js

@@ -158,17 +158,10 @@ PrivateKey._transformBuffer = function(buf, network) {
 
   info.network = Networks.get(buf[0], 'privatekey');
 
-  var allNetworks = Networks.all();
-  var matches = _.filter( allNetworks, function( network) {
-    return buf[0] === network.privatekey;
-  });
-
-  if (matches.length !== 1) {
+  if (!info.network) {
     throw new Error('Invalid network');
   }
 
-  info.network = matches[0];
-
   if (network && info.network !== Networks.get(network)) {
     throw new TypeError('Private key network mismatch');
   }