core/serialize: drop bogus deserialization of ipcns sockets

a70581ff added ExecRuntime.ipcns_storage_socket[], and
serialization in exec_runtime_serialize(), and deserialization in exec_runtime_deserialize_one(),
but also deserialization in exec_runtime_deserialize_compat(). exec_runtime_deserialize_compat()
is for deserializating ExecRuntime when it was serialized as part of the unit before
e8a565cb. There was never any code which would serialize
ExecRuntime.ipcns_storage_socket[] this way, so the deserialization attempts are pointless.