Remove package-template submodule

As reported on the list, this sub-module is causing issues because it causes several levels of recursive cloning that ends up requiring a username/password. I actually think that cloning astropy-helpers should now clone package-template by default, otherwise we end up with a circular git dependency. If we want to test the package-template in the astropy-helpers test suite, then we can clone it at test time if we really want. I'll see if there is an easy way to do that here.