FIX: remove refresh seconds override on cache_critical_dns (#16572)

This removes the option to override the sleep time between caching of
DNS records. The override was invalid because `''.to_i` is 0 in Ruby,
causing a tight loop calling the `run` method.

script/cache_critical_dns

@@ -23,7 +23,7 @@ CRITICAL_HOST_ENV_VARS = %w{
 HOST_RESOLVER_CACHE = {}
 HOST_HEALTHY_CACHE = {}
 HOSTS_PATH = ENV['DISCOURSE_DNS_CACHE_HOSTS_FILE'] || "/etc/hosts"
-REFRESH_SECONDS = ENV['DISCOURSE_DNS_CACHE_REFRESH_SECONDS'].to_i || 30
+REFRESH_SECONDS = 30
 
 module DNSClient
   def dns_client_with_timeout