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.

test/functional/test_runner.py

@@ -210,7 +210,6 @@ BASE_SCRIPTS = [
     'rpc_users.py',
     'rpc_whitelist.py',
     'feature_proxy.py',
-    'feature_syscall_sandbox.py',
     'wallet_signrawtransactionwithwallet.py --legacy-wallet',
     'wallet_signrawtransactionwithwallet.py --descriptors',
     'rpc_signrawtransactionwithkey.py',