plugins: do not call write_config() in plugins_interfaces()

Since we weaved the latter into the former, we can now get rid of
the transition code.  The behaviour doesn't change.

src/etc/inc/config.lib.inc

@@ -179,7 +179,7 @@ function write_config($desc = '', $backup = true)
 
     if (function_exists('plugins_interfaces')) {
         /* only pull plugins if plugins.inc was included before */
-        plugins_interfaces(false);
+        plugins_interfaces();
     }
 
     $cnf = OPNsense\Core\Config::getInstance();

src/etc/inc/plugins.inc

@@ -94,12 +94,11 @@ function plugins_syslog()
  * Every <plugin>_interface should return a named array containing the interface unique identifier and properties.
  *
  */
-function plugins_interfaces($write_allowed = true)
+function plugins_interfaces()
 {
     global $config;
 
     $stale_interfaces = array();
-    $write_required = false;
 
     // mark previous dynamic registrations stale
     if (isset($config['interfaces'])) {
@@ -135,7 +134,6 @@ function plugins_interfaces($write_allowed = true)
                     foreach ($intf_data as $prop_name => $prop_value) {
                         if ((empty($intf_config[$prop_name]) && !empty($prop_value)) || $intf_config[$prop_name] != $prop_value) {
                             $intf_config[$prop_name] = $prop_value;
-                            $write_required = true;
                         }
                     }
                 }
@@ -147,14 +145,8 @@ function plugins_interfaces($write_allowed = true)
     foreach ($stale_interfaces as $intf_ref => $no_data) {
         if (isset($config['interfaces'][$intf_ref])) {
             unset($config['interfaces'][$intf_ref]);
-            $write_required = true;
         }
     }
-
-    // configuration changed, materialize
-    if ($write_allowed && $write_required) {
-        write_config();
-    }
 }
 
 function plugins_configure()