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/scheduler.cpp

@@ -5,7 +5,6 @@
 #include <scheduler.h>
 
 #include <sync.h>
-#include <util/syscall_sandbox.h>
 #include <util/time.h>
 
 #include <cassert>
@@ -23,7 +22,6 @@ CScheduler::~CScheduler()
 
 void CScheduler::serviceQueue()
 {
-    SetSyscallSandboxPolicy(SyscallSandboxPolicy::SCHEDULER);
     WAIT_LOCK(newTaskMutex, lock);
     ++nThreadsServicingQueue;