calc/no_calc features complementary

[oletf]

I wanted to disable default features (to not have chrono serde time, nor calc), so added

duration-str = { version = "0.17.0", default-features = false }

and got a compile error in parsers::parse because I didn't add no_calc, so both blocks got disabled.

As both features are complementary, I thought that keeping only one would be less confusing. Docs mention no_calc so maybe it'd be the one to keep.

I've made a change on my fork, I can PR if it works for you!

[baoyachi]

Good job. 👍