Drop FnMut trait bounds from ExtractIf data structures

[dtolnay]

This is going to be required for resolving rust-lang/rust#137654.

Of the 4 distinct signatures shown in that issue, the one we'd like to standardize on is the one currently being used by alloc::collections::linked_list::ExtractIf, which is no F: Debug bound, no F: FnMut bound, only T: Debug bound.

Without this change in hashbrown, the desired Debug impls in std for std::collections::hash_map::ExtractIf and std::collections::hash_set::ExtractIf fail to compile.

error[E0277]: expected a `FnMut(&K, &mut V)` closure, found `F`
    --> library/std/src/collections/hash/map.rs:1683:11
     |
1683 |     base: base::ExtractIf<'a, K, V, F>,
     |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected an `FnMut(&K, &mut V)` closure, found `F`
     |
note: required by a bound in `hashbrown::hash_map::ExtractIf`
    --> hashbrown/src/map.rs:2601:8
     |
2599 | pub struct ExtractIf<'a, K, V, F, A: Allocator = Global>
     |            --------- required by a bound in this struct
2600 | where
2601 |     F: FnMut(&K, &mut V) -> bool,
     |        ^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `ExtractIf`
help: consider restricting type parameter `F` with trait `FnMut`
     |
1682 | pub struct ExtractIf<'a, K, V, F: for<'a, 'b> FnMut(&'a K, &'b mut V)> {
     |                                 +++++++++++++++++++++++++++++++++++++