examples/threaded-ssl: delete in favor of `examples/threaded`

examples/threaded-ssl: delete in favor of `examples/threaded`

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 61273f58125c41a84febe90a50238ee7fb12a9ad #20001
Follow-up to 971e8d661c68ce8859885c3ae865ff9441b62f0e #19526 #19524

Closes #20002