[libc++][NFC] Remove spurious check for is-constant-evaluated

This NFC change was reverted as part of 880fa7f, but the change is
really good regardless of the associated Clang patch.

Author: ldionne

libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/ranges.copy.segmented.pass.cpp

@@ -93,12 +93,10 @@ constexpr void test_join_view() {
 }
 
 int main(int, char**) {
-  if (!std::is_constant_evaluated()) {
-    test_containers<std::deque<int>, std::deque<int>>();
-    test_containers<std::deque<int>, std::vector<int>>();
-    test_containers<std::vector<int>, std::deque<int>>();
-    test_containers<std::vector<int>, std::vector<int>>();
-  }
+  test_containers<std::deque<int>, std::deque<int>>();
+  test_containers<std::deque<int>, std::vector<int>>();
+  test_containers<std::vector<int>, std::deque<int>>();
+  test_containers<std::vector<int>, std::vector<int>>();
 
   types::for_each(types::forward_iterator_list<int*>{}, []<class Iter> {
     test_join_view<Iter, Iter>();