Prior to this patch code used either `HAVE_PTHREAD_H`, or
`HAVE_THREADS_POSIX`, or both, to decide if POSIX Threads support is
present. In effect requiring both to be defined for a consistent build.
Drop detecting and guarding for `pthread.h`, and assume it present when
`HAVE_THREADS_POSIX` is set.
OS/400 had `HAVE_PTHREAD_H` set, but not `HAVE_THREADS_POSIX`, which
possibly left threading disabled in most sources.
Ref: https://pubs.opengroup.org/onlinepubs/009695399/basedefs/pthread.h.html
Ref: 930f2e8227#21144Closes#21158
After applying a recent fix made to `threaded.c` (formerly
`multithread.c`) to `threaded-ssl.c`, syncing and updating comments,
the two examples turned out to be identical except their test URLs.
Delete one of them to avoid duplication.
Also:
- examples/threaded: scope a variable.
- examples/threaded: merge comments from its deleted sibling.
Follow-up to 61273f5812#20001
Follow-up to 971e8d661c#19526#19524Closes#20002
