fix(redundant import)

Fixes

    Checking srml-grandpa v2.0.0 (/home/niklasad1/Github/substrate/srml/grandpa)
warning: the item `HashSet` is imported redundantly
   --> core/network/src/sync/extra_requests.rs:307:7
    |
17  | use std::collections::{HashMap, HashSet, VecDeque};
    |                                 ------- the item `HashSet` is already imported here
...
307 |         use std::collections::HashSet;
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: #[warn(unused_imports)] on by default