drop some unnecessary type constraints

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.06% :tada:

Comparison is base (6dc5133) 74.12% compared to head (0af7a6e) 74.18%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #96 +/- ## ========================================== + Coverage 74.12% 74.18% +0.06% ========================================== Files 17 17 Lines 827 829 +2 ========================================== + Hits 613 615 +2 Misses 214 214 ``` | [Files Changed](https://app.codecov.io/gh/ralexstokes/ssz-rs/pull/96?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Stokes) | Coverage Δ | | |---|---|---| | [ssz-rs/src/list.rs](https://app.codecov.io/gh/ralexstokes/ssz-rs/pull/96?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Stokes#diff-c3N6LXJzL3NyYy9saXN0LnJz) | `77.10% <ø> (ø)` | | | [ssz-rs/src/vector.rs](https://app.codecov.io/gh/ralexstokes/ssz-rs/pull/96?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Alex+Stokes#diff-c3N6LXJzL3NyYy92ZWN0b3IucnM=) | `83.13% <100.00%> (+0.41%)` | :arrow_up: |

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