Delete dead code

The references set seems to have been unused since `LegacySSHStore`
references were first created in
caa5793b4a.

The method decls never were upstream, and accidentally added by me in
062533f7cd (probably due to `git rerere`).
Sorry!

This reduces the diff from #3746.

src/libstore/legacy-ssh-store.cc

@@ -134,7 +134,6 @@ struct LegacySSHStore : public virtual LegacySSHStoreConfig, public virtual Stor
             /* Hash will be set below. FIXME construct ValidPathInfo at end. */
             auto info = std::make_shared<ValidPathInfo>(path, Hash::dummy);
 
-            PathSet references;
             auto deriver = readString(conn->from);
             if (deriver != "")
                 info->deriver = parseStorePath(deriver);