Provide kotlin.AssertionError in kotlin-stdlib-common

Remove related workarounds from kotlin-test-common

libraries/kotlin.test/common/src/main/kotlin/kotlin/test/DefaultAsserter.kt

@@ -11,7 +11,4 @@ class DefaultAsserter() : Asserter {
         else
             throw AssertionError(message)
     }
-}
-
-header fun AssertionError(message: String): Throwable
-header fun AssertionError(): Throwable
+}