test: remove test-v8-untrusted-code-mitigations

Untrusted code mitigations have now been removed from V8.

PR-URL: #40178
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Author: rmcilroy