core/serialize: drop bogus deserialization of ipcns sockets

core/serialize: drop bogus deserialization of ipcns sockets

a70581ffb5c13c91c76ff73ba6f5f3ff59c5a915 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
e8a565cb660a7a11f76180fe441ba8e4f9383771. There was never any code which would serialize
ExecRuntime.ipcns_storage_socket[] this way, so the deserialization attempts are pointless.