remove extraneous `num_elements` specialization

[jlchan]

@ranocha not sure why https://github.com/jlchan/StartUpDG.jl/pull/120 is failing, but this seems to achieve the same result without Windows CI failing.

[codecov[bot]]

Codecov Report

Merging #121 (9645043) into main (fbc55b9) will decrease coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
- Coverage   96.19%   96.19%   -0.01%     
==========================================
  Files          25       25              
  Lines        2842     2837       -5     
==========================================
- Hits         2734     2729       -5     
  Misses        108      108              

Impacted Files	Coverage Δ
src/MeshData.jl	95.13% <100.00%> (+0.51%)	:arrow_up:

... and 2 files with indirect coverage changes

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