remove the import of assign and ref.
remove the import of assign and ref.

They are no longer available in Base, and they are not used anywhere in the package.

zhmz90/Optim.jl,KristofferC/Optim.jl,matthieugomez/Optim.jl,JuliaPackageMirrors/Optim.jl,pkofod/Optim.jl,matthieugomez/Optim.jl,zhmz90/Optim.jl,KristofferC/Optim.jl,pkofod/Optim.jl,yuyichao/Optim.jl,yuyichao/Optim.jl,JuliaPackageMirrors/Optim.jl