cleanup: remove code to load w_*.zym weapon models;

cleanup: remove code to load w_*.zym weapon models;
instead make the loading of the v_ model only happen if the h_ model defines the "weapon" tag.
That way, the h_ model can be both an "invisible hand" model, or an old-style "full weapon" model.
Also, note that the engine loads ZYM models properly as ZYM even if renamed to dpm, so there is no need to try all extensions.

All in all, this saves quite some file open calls and should help with dedicated server performance, especially on operating systems with slow file opening (e.g. Windows with a running on-access virus scanner).

git-svn-id: svn://svn.icculus.org/nexuiz/trunk@6186 f962a42d-fe04-0410-a3ab-8c8b0445ebaa