Don't remove `Element` on `destroy()`

https://github.com/feross/p2p-graph/blob/3334451e147e77778200412f553f2cd1277a2750/index.js#L261

When calling `destroy()`, the `Element` where the graph is being applied is removed from the DOM. Since this element is not created and managed by `p2p-graph` but instead provided by us, it should be left untouched, as it's done at https://github.com/Novage/p2p-media-loader/blob/85adce9b0edbc00dfa073321d93a61590af3afc8/p2p-media-loader-demo/index.html#L1234-L1240