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/index/base.cpp

@@ -14,7 +14,6 @@
 #include <node/interface_ui.h>
 #include <shutdown.h>
 #include <tinyformat.h>
-#include <util/syscall_sandbox.h>
 #include <util/thread.h>
 #include <util/translation.h>
 #include <validation.h> // For g_chainman
@@ -167,7 +166,6 @@ static const CBlockIndex* NextSyncBlock(const CBlockIndex* pindex_prev, CChain&
 
 void BaseIndex::ThreadSync()
 {
-    SetSyscallSandboxPolicy(SyscallSandboxPolicy::TX_INDEX);
     // Wait for a possible reindex-chainstate to finish until continuing
     // with the index sync
     while (!g_indexes_ready_to_sync) {