Fix bug in which old mipstarts are not deleted

[odow]

Closes #352

[codecov]

Codecov Report

Base: 91.27% // Head: 91.44% // Increases project coverage by +0.17% 🎉

Coverage data is based on head (b9f50e5) compared to base (e05976a).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #422      +/-   ##
==========================================
+ Coverage   91.27%   91.44%   +0.17%     
==========================================
  Files           5        5              
  Lines        1937     1941       +4     
==========================================
+ Hits         1768     1775       +7     
+ Misses        169      166       -3     

Impacted Files	Coverage Δ
src/MOI/MOI_wrapper.jl	92.33% <100.00%> (+0.19%)	⬆️

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