minor cleanup - removed obsolete instanceof PString check

Author: tomasstupka

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/PythonAbstractObject.java

@@ -587,8 +587,6 @@ private static void addKeysFromObject(HashSet<String> keys, PythonObject o, bool
             String strKey;
             if (k instanceof String) {
                 strKey = (String) k;
-            } else if (k instanceof PString) {
-                strKey = ((PString) k).getValue();
             } else {
                 continue;
             }