fix: fix compilation warnings when running only with default features

[morenol]

I got:

warning: unused import: `itertools::Itertools`
 --> /Users/morenol/me/smartcore/src/algorithm/neighbour/fastpair.rs:2:5
  |
2 | use itertools::Itertools;
  |     ^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

which is only used in force-brute implementation

[Mec-iS]

thanks for this.