Remove nix-setuid-helper

AFAIK, nobody uses it, it's not maintained, and it has no tests.

src/libstore/local-store.hh

@@ -322,7 +322,7 @@ typedef set<Inode> InodesSeen;
    - the permissions are set of 444 or 555 (i.e., read-only with or
      without execute permission; setuid bits etc. are cleared)
    - the owner and group are set to the Nix user and group, if we're
-     in a setuid Nix installation. */
+     running as root. */
 void canonicalisePathMetaData(const Path & path, uid_t fromUid, InodesSeen & inodesSeen);
 void canonicalisePathMetaData(const Path & path, uid_t fromUid);
 
@@ -330,16 +330,4 @@ void canonicaliseTimestampAndPermissions(const Path & path);
 
 MakeError(PathInUse, Error);
 
-/* Whether we are root. */
-bool amPrivileged();
-
-/* Recursively change the ownership of `path' to the current uid. */
-void getOwnership(const Path & path);
-
-/* Like deletePath(), but changes the ownership of `path' using the
-   setuid wrapper if necessary (and possible). */
-void deletePathWrapped(const Path & path, unsigned long long & bytesFreed);
-
-void deletePathWrapped(const Path & path);
-
 }