Remove warning in the latest nightly

[mcarton]

We have a warning with the latest nightly:

warning: use of deprecated item: use `DefaultHasher` instead, #[warn(deprecated)] on by default
 --> clippy_lints/src/utils/hir.rs:4:31
  |
4 | use std::hash::{Hash, Hasher, SipHasher};
  |                               ^^^^^^^^^

warning: use of deprecated item: use `DefaultHasher` instead, #[warn(deprecated)] on by default
   --> clippy_lints/src/utils/hir.rs:275:8
    |
275 |     s: SipHasher,
    |        ^^^^^^^^^

warning: use of deprecated item: use `DefaultHasher` instead, #[warn(deprecated)] on by default
   --> clippy_lints/src/utils/hir.rs:282:16
    |
282 |             s: SipHasher::new(),
    |                ^^^^^^^^^^^^^^

See rust-lang/rust#36815.

It's probably not worth bumping the version for just that but it's kind of paradoxical to have warnings in a linter 😊