setup: remove Nevow 0.9.18 tarball from misc/dependencies

setup: remove Nevow 0.9.18 tarball from misc/dependencies
The 0.9.18 version of Nevow doesn't declare its dependency on Twisted in a machine-readable way ( http://divmod.org/trac/ticket/2629 ).  Neither does the current release of Nevow (0.9.31), but hopefully a future release of Nevow in the near future will fix this.
Also, we're going to be managing external dependencies like this in a separate darcs repository in the future instead of checking them into our Tahoe source tree.