Support deletion of SOC variables

[odow]

Unresolved TODO from #261

https://github.com/JuliaOpt/CPLEX.jl/blob/7c4acf18e78e360bb4691fe97b47d2e98d6d29c8/test/MathOptInterface/MOI_wrapper.jl#L48-L50

[odow]

An alternative is to remove SOC support in favor of MOI.Bridges.Constraint.SOCtoNonConvexQuadBridge.

[odow]

Closing for the same reason as jump-dev/Gurobi.jl#354 (comment)