[libc++] Remove invalid C++20 code from a test.

During the review of D91986 it has been discovered the in C++11
deprecated `throw()` exception specification has been removed in
C++20. Removed the part of the test code using this feature.

Author: mordante

libcxx/test/std/concepts/concept.destructible/destructible.compile.pass.cpp

@@ -34,12 +34,6 @@ struct NoexceptFalse {
   ~NoexceptFalse() noexcept(false);
 };
 
-// Since C++17 dynamic exception specifications are no longer
-// part of the standard.
-struct Throw {
-  ~Throw() throw();
-};
-
 struct Protected {
 protected:
   ~Protected() = default;
@@ -69,8 +63,6 @@ void test() {
   test<NoexceptTrue>();
   test<NoexceptFalse>();
 
-  test<Throw>();
-
   test<Protected>();
   test<Private>();