remove third-party directory

Please remove this third-party directory, create a INSTALL file that lists dependencies instead.

Like this:

pacman -S libuv luajit

Having a third-party directory could create problems in the future when a contributor sends you a pull requests that makes changes to the bundled libraries, then the problem will fail to compile with system libraries, thus creating problems for Linux distributions to ship neovim in their repositories.

I can name other projects if you want that went in this same direction and I also can point you out to the Debian bug tracker where they refuse to ship the project completely due to this reason.

Thanks.