Fix: Remove PString specialization in PyObjectStrAsJavaStringNode.

As PString subclasses can overwrite `__str__` (which is demonstrated for example in
`test_unicode.py#test_format_subclass`) we need to check `__str__` for PString instances, too,
and have to rely on the generic implementation for them.

Author: phoeinx

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/lib/PyObjectStrAsJavaStringNode.java

@@ -40,7 +40,6 @@
  */
 package com.oracle.graal.python.lib;
 
-import com.oracle.graal.python.builtins.objects.str.PString;
 import com.oracle.graal.python.nodes.PNodeWithContext;
 import com.oracle.graal.python.nodes.util.CannotCastException;
 import com.oracle.graal.python.nodes.util.CastToJavaStringNode;
@@ -72,16 +71,6 @@ static String doString(String obj) {
         return obj;
     }
 
-    @Specialization
-    static String doPString(PString obj,
-                    @Cached CastToJavaStringNode castToString) {
-        try {
-            return castToString.execute(obj);
-        } catch (CannotCastException e) {
-            throw CompilerDirectives.shouldNotReachHere("PString result not convertible to string");
-        }
-    }
-
     @Specialization
     static String doGeneric(VirtualFrame frame, Object obj,
                     @Cached PyObjectStrAsObjectNode strNode,