what if you just delete the Warbird/Dderidex plugin (both .py and .pyc) and make a new one from scratch?
every time I delete a plugin, the ship stops showing up. that's the way it's supposed to be, isn't it?
an afterthought: are you sure you have just one version of the warbird? maybe another plugin was meant to overwrite the stock Warbird and now you're creating a duplicate of the original as Dderidex, but the second plugin keeps making an entry named "Warbird" (if that's even possible)
if you want the ship to be named "Dderidex", modify the "longname" field. also, if you want it to show up as "Dderidex" in game, modify the name in the ship's file (scripts\ships\Warbird.py)