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/feature_startupnotify.py

@@ -18,7 +18,6 @@ FILE_NAME = "test.txt"
 class StartupNotifyTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 1
-        self.disable_syscall_sandbox = True
 
     def run_test(self):
         tmpdir_file = os.path.join(self.options.tmpdir, NODE_DIR, FILE_NAME)