Code cleanup after #232

[gaurav-arya]

There are a couple of code cleanup tasks that will be left behind by #232:

• [ ] Remove code repetition in https://github.com/JuliaDiff/SparseDiffTools.jl/blob/master/test/test_jaches_products.jl and https://github.com/JuliaDiff/SparseDiffTools.jl/blob/master/test/test_vecjac_products.jl by refactoring into a function in the test utilities
• [ ] Refactor all the boilerplate functions for FwdModeAutoDiffVecProd and friends into the use of a FunctionOperator (may need https://github.com/SciML/SciMLOperators.jl/issues/57)
• [ ] Support unused params in FunctionOperators (requires change to SciMLOps) and deprecate the use of p and t args in VecJac and friends since we don't need them

This issue tracks these so we don't forget about them 🙂