[time.duration] repeats "duration" in subclauses

[JohelEGP]

[time.duration] doesn't follow this specification guideline:

Do not repeat parts of subclause headings in nested headings (decided in #1242). Good: "Class template tuple" / "Construction". Bad: "Class template vector" / "vector capacity".

You can see the repetition in almost all of the subclauses' headings at http://eel.is/c++draft/#utilities:

[time.duration.cast] and [time.duration.literals] are fine, the later based on the specification of literals elsewhere in the standard following this pattern.

[jensmaurer]

We'll move [time] into a separate top-level subclause; see #1919. To avoid merge conflicts, this issue is on hold until that subclause has materialized.