[ASan] Remove the declarations of pthread_workqueue_t and pthread_workitem_handle_t, which are used no more.
Kudos to Jeremy Huddleston Sequoia <jeremyhu@apple.com>


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@173143 91177308-0d34-0410-b5e6-96231b3b80d8