J2K: got rid from Converter.convertElement + fixed local classes conversion

#KT-5202 Fixed

j2k/tests/testData/ast/nullability/MethodReturnsNullInLocalClass.java

@@ -1,9 +1,8 @@
-//file
 class C {
     String foo() {
         class Local {
             String foo() { return null; }
-        };
+        }
         new Local().foo();
         return "";
     }

j2k/tests/testData/ast/nullability/MethodReturnsNullInLocalClass.kt

@@ -0,0 +1,11 @@
+class C {
+    fun foo(): String {
+        class Local {
+            fun foo(): String? {
+                return null
+            }
+        }
+        Local().foo()
+        return ""
+    }
+}