Remove the ValidPathInfo == operator

It is dead code. It was added in
8e0946e8df as part of the repeated /
enforce-determinism feature, but that was removed in
8fdd156a65.

It is not good because it skips many fields. For testing purposes we
will soon want to add a new one that doesn't skip fields, but we want to
make sure making == sensitive to those fields won't change how Nix
works. Proving in this commit that the old version is dead code achieves
that.

src/libstore/path-info.hh

@@ -72,14 +72,6 @@ struct ValidPathInfo
      */
     std::optional<ContentAddress> ca;
 
-    bool operator == (const ValidPathInfo & i) const
-    {
-        return
-            path == i.path
-            && narHash == i.narHash
-            && references == i.references;
-    }
-
     /**
      * Return a fingerprint of the store path to be used in binary
      * cache signatures. It contains the store path, the base-32