Fix warning about unused type parameter

[blegat]

During precompilation, I got

WARNING: method definition for projection_on_set at /home/blegat/.julia/packages/MathOptSetDistances/pO2hs/src/projections.jl:807 declares type variable R but does not use it.

By the way, why not completely remove the parameters since they are not used in the function ?

[codecov[bot]]

Codecov Report

Merging #69 (dc540bf) into master (470e508) will not change coverage. The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master      #69   +/-   ##
=======================================
  Coverage   86.73%   86.73%           
=======================================
  Files           6        6           
  Lines         671      671           
=======================================
  Hits          582      582           
  Misses         89       89           

Impacted Files	Coverage Δ
src/projections.jl	88.20% <0.00%> (ø)

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[matbesancon]

By the way, why not completely remove the parameters since they are not used in the function ?

for annotation purposes mostly