refactor: remove unnecessary unsafe code

[robjtede]

There's absolutely no performance win from doing it.

static_literal unwrap   time:   [2.6493 µs 2.6519 µs 2.6549 µs]
Found 5 outliers among 100 measurements (5.00%)
  5 (5.00%) high mild

static_literal unwrap_unchecked
                        time:   [2.7218 µs 2.7237 µs 2.7255 µs]
Found 6 outliers among 100 measurements (6.00%)
  5 (5.00%) high mild
  1 (1.00%) high severe

static_distance_tree unwrap
                        time:   [552.33 ns 552.96 ns 553.65 ns]
Found 7 outliers among 100 measurements (7.00%)
  5 (5.00%) high mild
  2 (2.00%) high severe

static_distance_tree unwrap_unchecked
                        time:   [623.83 ns 624.40 ns 624.96 ns]
Found 3 outliers among 100 measurements (3.00%)
  2 (2.00%) high mild
  1 (1.00%) high severe

[anatawa12]

LGTM. Thank you!