Remove the syscall sandbox

After initially being merged in #20487, it's no-longer clear that an
internal syscall sandboxing mechanism is something that Bitcoin Core
should have/maintain, especially when compared to better
maintained/supported alterantives, i.e firejail.

Note that given where it's used, the sandbox also gets dragged into the
kernel.

There is some related discussion in #24771.

This should not require any sort of deprecation, as this was only ever
an opt-in, experimental feature.

Closes #24771.

src/checkqueue.h

@@ -7,7 +7,6 @@
 
 #include <sync.h>
 #include <tinyformat.h>
-#include <util/syscall_sandbox.h>
 #include <util/threadnames.h>
 
 #include <algorithm>
@@ -149,7 +148,6 @@ public:
         for (int n = 0; n < threads_num; ++n) {
             m_worker_threads.emplace_back([this, n]() {
                 util::ThreadRename(strprintf("scriptch.%i", n));
-                SetSyscallSandboxPolicy(SyscallSandboxPolicy::VALIDATION_SCRIPT_CHECK);
                 Loop(false /* worker thread */);
             });
         }