Use local default for postgres host in node streaming service (#3240)

This location varies across postgres installations, and it seems like the pg
package knows how to guess correctly on each system.

streaming/index.js

@@ -86,7 +86,6 @@ if (cluster.isMaster) {
   const pgConfigs = {
     development: {
       database: 'mastodon_development',
-      host:     '/var/run/postgresql',
       max:      10,
     },