Fix evaluation cache

98e361ad4c introduced a regression where
previously stored attributes were replaced by placeholders. As a
result, a command like 'nix build nixpkgs#hello' had to be executed at
least twice to get caching.

This code does not seem necessary for suggestions to work.

src/libexpr/eval-cache.cc

@@ -507,11 +507,6 @@ std::shared_ptr<AttrCursor> AttrCursor::maybeGetAttr(Symbol name, bool forceErro
         return nullptr;
         //throw TypeError("'%s' is not an attribute set", getAttrPathStr());
 
-    for (auto & attr : *v.attrs) {
-        if (root->db)
-            root->db->setPlaceholder({cachedValue->first, attr.name});
-    }
-
     auto attr = v.attrs->get(name);
 
     if (!attr) {