Return MIP gap from gurobi, don't calculate it

[odow]

https://github.com/JuliaOpt/Gurobi.jl/blob/ee3d837ea3c375b5dc2cca4dac61f6dd0a7541aa/src/MOI_wrapper.jl#L584-L588

Ref discussion in jump-dev/MathOptInterface.jl#590