[libc++] Remove obsolete guards for join_view being experimental (llvm#126697)

These TODOs were forgotten when join_view was made non-experimental. By
removing these checks, we slightly increase the coverage of the test
suite.

Author: ldionne

libcxx/test/std/algorithms/alg.nonmodifying/alg.find/find.pass.cpp

@@ -215,9 +215,7 @@ TEST_CONSTEXPR_CXX20 bool test() {
   Test<TriviallyComparable<wchar_t>, TriviallyComparable<wchar_t>>().operator()<TriviallyComparable<wchar_t>*>();
 #endif
 
-  // TODO: Remove the `_LIBCPP_ENABLE_EXPERIMENTAL` check once we have the FTM guarded or views::join isn't
-  // experimental anymore
-#if TEST_STD_VER >= 20 && (!defined(_LIBCPP_VERSION) || defined(_LIBCPP_ENABLE_EXPERIMENTAL))
+#if TEST_STD_VER >= 20
   {
     std::vector<std::vector<int>> vec = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
     auto view                         = vec | std::views::join;

libcxx/test/std/algorithms/alg.nonmodifying/alg.find/ranges.find.pass.cpp

@@ -131,9 +131,7 @@ constexpr bool test() {
                     });
                   });
 
-  // TODO: Remove the `_LIBCPP_ENABLE_EXPERIMENTAL` check once we have the FTM guarded or views::join isn't
-  // experimental anymore
-#if TEST_STD_VER >= 20 && (!defined(_LIBCPP_VERSION) || defined(_LIBCPP_ENABLE_EXPERIMENTAL))
+#if TEST_STD_VER >= 20
   {
     std::vector<std::vector<int>> vec = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
     auto view                         = vec | std::views::join;

libcxx/test/std/algorithms/alg.nonmodifying/alg.foreach/for_each.pass.cpp

@@ -56,9 +56,7 @@ struct Test {
 TEST_CONSTEXPR_CXX20 bool test() {
   types::for_each(types::cpp17_input_iterator_list<int*>(), Test());
 
-  // TODO: Remove the `_LIBCPP_ENABLE_EXPERIMENTAL` check once we have the FTM guarded or views::join isn't
-  // experimental anymore
-#if TEST_STD_VER >= 20 && defined(_LIBCPP_ENABLE_EXPERIMENTAL)
+#if TEST_STD_VER >= 20
   { // Make sure that the segmented iterator optimization works during constant evaluation
     std::vector<std::vector<int>> vecs = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
     auto v                             = std::views::join(vecs);