remove unused type variable

[ranocha]

I found this in our CI tests of Trixi.jl at https://github.com/trixi-framework/Trixi.jl/actions/runs/3805067166/jobs/6472726862#step:7:7143

[codecov]

Codecov Report

Merging #20 (66f534b) into master (809383c) will decrease coverage by 2.76%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
- Coverage   85.37%   82.60%   -2.77%     
==========================================
  Files           8        8              
  Lines         253      253              
==========================================
- Hits          216      209       -7     
- Misses         37       44       +7     

Flag	Coverage Δ
unittests	82.60% <0.00%> (-2.77%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/onehot.jl	79.48% <0.00%> (ø)
src/core.jl	58.33% <0.00%> (-12.51%)	⬇️
src/sets.jl	77.27% <0.00%> (-4.55%)	⬇️
src/dicts.jl	93.10% <0.00%> (-3.45%)	⬇️

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[jishnub]

Duplicate of #18