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/node/blockstorage.cpp

@@ -19,7 +19,6 @@
 #include <undo.h>
 #include <util/batchpriority.h>
 #include <util/fs.h>
-#include <util/syscall_sandbox.h>
 #include <validation.h>
 
 #include <map>
@@ -869,7 +868,6 @@ public:
 
 void ThreadImport(ChainstateManager& chainman, std::vector<fs::path> vImportFiles, const fs::path& mempool_path)
 {
-    SetSyscallSandboxPolicy(SyscallSandboxPolicy::INITIALIZATION_LOAD_BLOCKS);
     ScheduleBatchPriority();
 
     {

src/node/interfaces.cpp

@@ -94,7 +94,7 @@ public:
     bool baseInitialize() override
     {
         if (!AppInitBasicSetup(args(), Assert(context())->exit_status)) return false;
-        if (!AppInitParameterInteraction(args(), /*use_syscall_sandbox=*/false)) return false;
+        if (!AppInitParameterInteraction(args())) return false;
 
         m_context->kernel = std::make_unique<kernel::Context>();
         if (!AppInitSanityChecks(*m_context->kernel)) return false;