MDL-44323 admin: use correct css class for warning

The notifyproblem notification is consistent with the rest of
the warnings we report.

admin/roles/assign.php

@@ -176,7 +176,7 @@ if ($roleid) {
     // Show UI for assigning a particular role to users.
     // Print a warning if we are assigning system roles.
     if ($context->contextlevel == CONTEXT_SYSTEM) {
-        echo $OUTPUT->box(get_string('globalroleswarning', 'core_role'));
+        echo $OUTPUT->notification(get_string('globalroleswarning', 'core_role'));
     }
 
     // Print the form.
@@ -251,7 +251,7 @@ if ($roleid) {
 
     // Print a warning if we are assigning system roles.
     if ($context->contextlevel == CONTEXT_SYSTEM) {
-        echo $OUTPUT->box(get_string('globalroleswarning', 'core_role'));
+        echo $OUTPUT->notification(get_string('globalroleswarning', 'core_role'));
     }
 
     // Print instruction.

theme/bootstrapbase/less/moodle/admin.less

@@ -747,12 +747,6 @@ img.iconsmall {
     padding-bottom: 0;
 }
 
-// Hacks for badly marked up admin warnings.
-#page-admin-roles-assign div.box.generalbox {
-    .alert;
-    .alert-danger;
-}
-
 .maintenancewarning {
     padding: 3px 1em;
     text-align: center;