Get rid of filterDrvOutputs

We don't need it any more, because we only used it in the
single-wanted-output `DerivationGoal`.

src/libstore/realisation.cc

@@ -135,18 +135,6 @@ size_t Realisation::checkSignatures(const PublicKeys & publicKeys) const
     return good;
 }
 
-SingleDrvOutputs filterDrvOutputs(const OutputsSpec & wanted, SingleDrvOutputs && outputs)
-{
-    SingleDrvOutputs ret = std::move(outputs);
-    for (auto it = ret.begin(); it != ret.end();) {
-        if (!wanted.contains(it->first))
-            it = ret.erase(it);
-        else
-            ++it;
-    }
-    return ret;
-}
-
 StorePath RealisedPath::path() const
 {
     return std::visit([](auto && arg) { return arg.getPath(); }, raw);