Make nix store delete ./result work

[thufschmitt]

Calling nix store delte on a result (or equivalent) symlink is a fairly intuitive thing to do, but doesn't work (since that result
symlink makes its target a GC root).

Proposal: get it to work by making nix store delete ./result first remove the symlink and then do the deletion.

(Pulled out of #7701 since it's a nice improvement, but not in the critical path of the stabilisation).