init: Remove Shutdown() node.args reset

This commit removes the `node.args = nullptr` assignment in the Shutdown()
function.

Clearing node.args there never made sense because it made the
Shutdown() function not idempotent, making it fragile and causing issues like
https://github.com/bitcoin/bitcoin/issues/23186.

The assignment also causes segfaults in GUI unit tests when a new
node().initParameterInteraction() call is added in OptionsModel to apply to Qt
settings (happens because AppTests calls Shutdown() which sets node.args to
null, and OptionTests runs after AppTests and then needs node.args not to be
null.)

src/init.cpp

@@ -320,7 +320,6 @@ void Shutdown(NodeContext& node)
         LogPrintf("%s: Unable to remove PID file: %s\n", __func__, fsbridge::get_filesystem_error_message(e));
     }
 
-    node.args = nullptr;
     LogPrintf("%s: done\n", __func__);
 }