Remove unused type variables

[anirudh2]

Just removing type variables N that seem to be unused.

[codecov[bot]]

Codecov Report

Base: 69.41% // Head: 69.41% // No change to project coverage :thumbsup:

Coverage data is based on head (faeaa7c) compared to base (0b86a6a). Patch coverage: 60.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #107 +/- ## ======================================= Coverage 69.41% 69.41% ======================================= Files 7 7 Lines 909 909 ======================================= Hits 631 631 Misses 278 278 ``` | [Impacted Files](https://codecov.io/gh/JuliaData/TypedTables.jl/pull/107?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaData) | Coverage Δ | | |---|---|---| | [src/DictTable.jl](https://codecov.io/gh/JuliaData/TypedTables.jl/pull/107/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaData#diff-c3JjL0RpY3RUYWJsZS5qbA==) | `57.78% <60.00%> (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=JuliaData). 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=JuliaData)

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