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/wallet_signer.py

@@ -45,9 +45,6 @@ class WalletSignerTest(BitcoinTestFramework):
 
     def set_test_params(self):
         self.num_nodes = 2
-        # The experimental syscall sandbox feature (-sandbox) is not compatible with -signer (which
-        # invokes execve).
-        self.disable_syscall_sandbox = True
 
         self.extra_args = [
             [],