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