remove unbound type parameter

[nsajko]

I didn't check, but unbound type parameters often cause performance issues, so this may not be merely cosmetic.

[codecov[bot]]

Codecov Report

Base: 86.01% // Head: 86.01% // No change to project coverage :thumbsup:

Coverage data is based on head (87b6c74) compared to base (037523e). Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #117 +/- ## ======================================= Coverage 86.01% 86.01% ======================================= Files 31 31 Lines 2496 2496 ======================================= Hits 2147 2147 Misses 349 349 ``` | [Impacted Files](https://codecov.io/gh/JuliaApproximation/DomainSets.jl/pull/117?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/domains/cube.jl](https://codecov.io/gh/JuliaApproximation/DomainSets.jl/pull/117/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL2RvbWFpbnMvY3ViZS5qbA==) | `96.50% <0.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation)

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[daanhb]

That must have been a typo, thanks for the fix