Drop usage of deprecated Any?.freeze() in K/N target (#2819)

The legacy memory model was removed from the K/N runtime in 1.9.20.
Now, this function does nothing, and its usages can be safely dropped.

Worker.execute functions no longer require the argument to be frozen,
as objects can no longer be frozen.

formats/json-tests/nativeTest/src/kotlinx/serialization/json/MultiWorkerJsonTest.kt

@@ -22,7 +22,7 @@ class MultiWorkerJsonTest {
                 assertEquals(PlainOne(42), json().decodeFromString("""{"one":42,"two":239}"""))
             }
         }
-        worker.executeAfter(1000, operation.freeze())
+        worker.executeAfter(1000, operation)
         for (i in 0..999) {
             assertEquals(PlainTwo(239), json().decodeFromString("""{"one":42,"two":239}"""))
         }