rm unused DiffEqOperators dependency

[vpuri3]

Sundials.jl on  master is 📦 v4.14.0 via ஃ v1.8.5 took 3s 
❯ grep -Irni diffeqoper
./Project.toml:32:DiffEqOperators = "9fdde737-9c7f-55bf-ade8-46b3f136cc48"
./Project.toml:42:test = ["Test", "AlgebraicMultigrid", "ODEProblemLibrary", "DAEProblemLibrary", "ForwardDiff", "DiffEqOperators", "SparseDiffTools", "SparsityTracing", "IncompleteLU", "ModelingToolkit"]

[codecov]

Codecov Report

Merging #390 (b476fcd) into master (fe412b3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #390   +/-   ##
=======================================
  Coverage   78.26%   78.26%           
=======================================
  Files          11       11           
  Lines        1463     1463           
=======================================
  Hits         1145     1145           
  Misses        318      318           

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