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/mapport.cpp

@@ -14,7 +14,6 @@
 #include <net.h>
 #include <netaddress.h>
 #include <netbase.h>
-#include <util/syscall_sandbox.h>
 #include <util/thread.h>
 #include <util/threadinterrupt.h>
 
@@ -219,7 +218,6 @@ static bool ProcessUpnp()
 
 static void ThreadMapPort()
 {
-    SetSyscallSandboxPolicy(SyscallSandboxPolicy::INITIALIZATION_MAP_PORT);
     bool ok;
     do {
         ok = false;