Revert "[libc++] Fix build failure with libdispatch backend (#141606)"

It turns out that we're about to revert the patches that added these
annotations in the first place. I was too hasty to land this.

This reverts commit 56594710c7.

libcxx/src/pstl/libdispatch.cpp

@@ -12,7 +12,6 @@
 #include <dispatch/dispatch.h>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
-_LIBCPP_BEGIN_EXPLICIT_ABI_ANNOTATIONS
 namespace __pstl::__libdispatch {
 
 void __dispatch_apply(size_t chunk_count, void* context, void (*func)(void* context, size_t chunk)) noexcept {
@@ -30,5 +29,4 @@ __chunk_partitions __partition_chunks(ptrdiff_t element_count) noexcept {
 }
 
 } // namespace __pstl::__libdispatch
-_LIBCPP_END_EXPLICIT_ABI_ANNOTATIONS
 _LIBCPP_END_NAMESPACE_STD