Instead of having the extra symlink in the root, this puts the symlink
in the alacritty crate. Since we build the package on Linux this is not
a problem and even allows us to get rid of the `alacritty.png` symlink.

To avoid having complicated symlinks with regards to the windows build
assets directory, it has just been removed from the `extra` directory
completely. Since we only need it for building, it doesn't matter where
it's located and users will never have to interact with it manually
anyway.

Closes #6242.