siteVersionDir isn't a full path, it's just the name of the version
directory entry in $out/share/emacs, e.g. "27.1", so since
d1b0eef ("emacs: Don't use interpolation for version"), we've been
trying to remove a non-existent directory. This would have been
caught if we hadn't been giving -f to rm unnecessarily, because -f
suppresses errors. As well as fixing the path, I've removed the -f
from rm. Doing this to the line above as well revealed that rm-ing
$out/var was no longer necessary, since nothing has been put there
since ac23a7c ("emacs: 25.3 → 26.1") -- it would now only be
created if we set the --with-gameuser configure option, which we
don't.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.