Make ImmutableObject.toString deterministic (#1469)

* Make ImmutableObject.toString deterministic

Remove the identity hash from the output. There is no use case
(including debugging) for it.

Removing it allows us to also remove some overriding implementations in
subclasses, and may also simplify tests.

core/src/test/java/google/registry/model/ImmutableObjectTest.java

@@ -68,11 +68,8 @@ public class ImmutableObjectTest {
   @Test
   void testToString_simpleClass() {
     SimpleObject object = new SimpleObject("foo", null);
-    assertThat(object.toString()).isEqualTo(""
-        + "SimpleObject (@" + System.identityHashCode(object) + "): {\n"
-        + "    a=foo\n"
-        + "    b=null\n"
-        + "}");
+    assertThat(object.toString())
+        .isEqualTo("" + "SimpleObject: {\n" + "    a=foo\n" + "    b=null\n" + "}");
   }
 
   @Test
@@ -332,8 +329,8 @@ public class ImmutableObjectTest {
 
     // The hash code test test is implicit in "equals", it is added here just for clarity.
     assertThat(instance1.hashCode()).isEqualTo(instance2.hashCode());
-    assertThat(instance1.toString()).matches(
-        "(?s)HasInsignificantFields (.*): \\{\\s*significant=significant\\s*\\}\\s*");
+    assertThat(instance1.toString())
+        .matches("(?s)HasInsignificantFields: \\{\\s*significant=significant\\s*\\}\\s*");
   }
 
   static class HasInsignificantFields extends ImmutableObject {