src: Remove unused imports raised by linter

[qmonnet]

I observed the following warning while running the tests, let's address
them:

warning: the item `time` is imported redundantly
  --> src/helpers.rs:22:5
   |
22 | use time;
   |     ^^^^
   |
  ::: src/lib.rs:32:1
   |
32 | extern crate time;
   | ------------------ the item `time` is already imported here
   |
   = note: `#[warn(unused_imports)]` on by default

warning: the item `std` is imported redundantly
 --> src/jit.rs:8:5
  |
8 | use std;
  |     ^^^ the item `std` is already imported by prelude