Stop an empty box appearing after Notes on the network device form

CustomFieldsInputs renders nothing when a site has defined no custom fields,
which is the normal state. The network form was the only one wrapping it in a
fieldset, and a fieldset draws its border whether or not anything is inside, so
an empty bordered box sat below Notes on every new network device.

Machines, PCs, printers and measuring tools all place the component bare. This
now matches them.

plugins/network/frontend/views/NetworkDeviceForm.vue

@@ -254,10 +254,12 @@
           </div>
         </fieldset>
 
-        <!-- Site-defined custom fields for network devices -->
+        <!-- Site-defined custom fields for network devices.
-        <fieldset>
+             NOT wrapped in a fieldset: the component renders nothing when a site
+             has defined no custom fields, but a fieldset draws its border either
+             way, so an empty box appeared after Notes. Every other asset form
+             places this bare; this one was the exception. -->
         <CustomFieldsInputs ref="customFieldsRef" :assettypeid="NETWORK_ASSETTYPEID" :assetid="currentAssetId" />
-        </fieldset>
 
         <!-- Form Actions -->
         <div class="form-actions">