Clean up .gitignore.

Which ones do you guys think we actually need to keep? I always do out of tree builds, so it turns out, I never need any of these.

Note, you can use a .gitconfig entry to specific a local ignore file for things that are consistent across your machine, like *.swp for vi or ~* for emacs.
These two are the end product of a Mac package bundle build, may or may not be wanted to track - depends on whether the user is distributing binaries via GitHub I suppose.