remove unused import in test case

[striezel]

Removes a warning that occurs when running tests with the dates feature enabled. The warning can be seen in the Windows / AppVeyor builds, e.g. here: https://ci.appveyor.com/project/tafia/calamine/builds/41754320

warning: unused import: `Duration`
   --> src\datatype.rs:327:22
    |
327 |         use chrono::{Duration, NaiveDate, NaiveDateTime, NaiveTime};
    |                      ^^^^^^^^
    |
    = note: `#[warn(unused_imports)]` on by default
warning: `calamine` (lib test) generated 1 warning

[striezel]

@tafia I just rebased this against the current master branch. Any feedback on this pull request would be appreciated.