Rm warnings if cost of artificial vars not provided by the user

[guimarqu]

These warnings are misleading (cf #545 (comment))

[codecov]

Codecov Report

Merging #588 (355473a) into master (3c64445) will increase coverage by 0.15%.
The diff coverage is n/a.

❗ Current head 355473a differs from pull request most recent head e444478. Consider uploading reports for the commit e444478 to get more accurate results

@@            Coverage Diff             @@
##           master     #588      +/-   ##
==========================================
+ Coverage   86.65%   86.81%   +0.15%     
==========================================
  Files          47       47              
  Lines        4796     4748      -48     
==========================================
- Hits         4156     4122      -34     
+ Misses        640      626      -14     

Impacted Files	Coverage Δ
src/optimize.jl	93.54% <ø> (-0.40%)	⬇️
src/MathProg/varconstr.jl	86.90% <0.00%> (+0.61%)	⬆️
src/MOIwrapper.jl	85.78% <0.00%> (+1.51%)	⬆️

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