[chores] Do not allow selecting columns when exporting devices

The django-export-import dependency was upgraded in
openwisp-controller which now allows selecting the columns during
export operation.

Since this can cause issues during import of files with limited
fields, we decided to rollback to the old implementation which
exports all the fields of the device model.

Related openwisp/openwisp-controller#915

Author: pandafy

openwisp_monitoring/device/admin.py

@@ -14,6 +14,7 @@
 from django.utils.safestring import mark_safe
 from django.utils.translation import gettext_lazy as _
 from import_export.admin import ImportExportMixin
+from import_export.forms import ExportForm
 from nested_admin.nested import (
     NestedGenericStackedInline,
     NestedModelAdmin,
@@ -370,6 +371,7 @@ def get_inlines(self, request, obj=None):
 
 
 class DeviceAdminExportable(ImportExportMixin, DeviceAdmin):
+    export_form_class = ExportForm
     resource_class = DeviceMonitoringResource
     # Added to support both reversion and import-export
     change_list_template = 'admin/config/change_list_device.html'