refactor(edmonds-karp): drop two unsafe Send impls the compiler already provides

[tachsin]

Closes #817.

Removes both unsafe impl Send. Every field of SparseCapacity and DenseCapacity is Send whenever C is, so the compiler derives the same impl with the same bound, and the crate is left with no unsafe in this file at all.

Adds a compile-time assertion in its place:

const fn assert_send<T: Send>() {}
assert_send::<DenseCapacity<i32>>();
assert_send::<SparseCapacity<i32>>();

so the property is still verified, and a future field that is not Send fails the build instead of being silently promised away by the hand-written impl.

Checking

Purely a removal; no behaviour changes. 291 tests pass, clippy and rustfmt clean.

This touches the same file as #819 but not the same lines; I checked they merge in either order.