chore(crypto): remove unnecessary lifetimes in canister_threshold_sig test utils

[fspreiss]

Removes some unnecessary lifetimes in the canister_threshold_sig test utils to get rid of compiler warnings of the following form:

warning: elided lifetime has a name
   --> rs/crypto/test_utils/canister_threshold_sigs/src/lib.rs:822:35
    |
819 |         pub fn filter_by_receivers<'a, T: AsRef<IDkgReceivers> + 'a>(
    |                                    -- lifetime `'a` declared here
...
822 |         ) -> impl Iterator<Item = &Node> + 'a {
    |                                   ^ this elided lifetime gets resolved as `'a`
    |
    = note: `#[warn(elided_named_lifetimes)]` on by default