Faster deletion of bounds in fix

[blegat]

delete_upper_bound(variable) was going back to a case where we need to access the backend and get an allocation for type unstability.

[codecov]

Codecov Report

Merging #2521 (9d27af8) into master (54c8b58) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master    #2521   +/-   ##
=======================================
  Coverage   92.44%   92.44%           
=======================================
  Files          43       43           
  Lines        4696     4696           
=======================================
  Hits         4341     4341           
  Misses        355      355           

Impacted Files	Coverage Δ
src/variables.jl	98.47% <50.00%> (ø)

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