[libc++] Remove "clang-format off/on" comments. NFC.

These headers have stabilized; we don't expect anyone to be
blindly clang-formatting them anymore.
Leave the comments in `__format/*.h` for Mark to remove at his leisure.

libcxx/include/__iterator/concepts.h

@@ -28,8 +28,6 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if !defined(_LIBCPP_HAS_NO_RANGES)
 
-// clang-format off
-
 // [iterator.concept.readable]
 template<class _In>
 concept __indirectly_readable_impl =
@@ -259,8 +257,6 @@ concept indirectly_movable_storable =
 // Note: indirectly_swappable is located in iter_swap.h to prevent a dependency cycle
 // (both iter_swap and indirectly_swappable require indirectly_readable).
 
-// clang-format on
-
 #endif // !defined(_LIBCPP_HAS_NO_RANGES)
 
 _LIBCPP_END_NAMESPACE_STD

libcxx/include/__ranges/concepts.h

@@ -29,8 +29,6 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-// clang-format off
-
 #if !defined(_LIBCPP_HAS_NO_RANGES)
 
 namespace ranges {
@@ -126,8 +124,6 @@ namespace ranges {
 
 #endif // !defined(_LIBCPP_HAS_NO_RANGES)
 
-// clang-format on
-
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___RANGES_CONCEPTS_H

libcxx/include/__ranges/data.h

@@ -26,7 +26,6 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if !defined(_LIBCPP_HAS_NO_RANGES)
 
-// clang-format off
 namespace ranges {
 // [range.prim.data]
 namespace __data {
@@ -72,8 +71,6 @@ inline namespace __cpo {
 } // namespace __cpo
 } // namespace ranges
 
-// clang-format off
-
 #endif // !defined(_LIBCPP_HAS_NO_RANGES)
 
 _LIBCPP_END_NAMESPACE_STD

libcxx/include/__ranges/empty.h

@@ -24,7 +24,6 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if !defined(_LIBCPP_HAS_NO_RANGES)
 
-// clang-format off
 namespace ranges {
 // [range.prim.empty]
 namespace __empty {
@@ -72,7 +71,6 @@ inline namespace __cpo {
   inline constexpr auto empty = __empty::__fn{};
 } // namespace __cpo
 } // namespace ranges
-// clang-format off
 
 #endif // !defined(_LIBCPP_HAS_NO_RANGES)