remove exports of undefined add_∇ and add_∇!

[oxinabox]

They are not define, so probably should not be exported.

julia> Nabla.add_∇
ERROR: UndefVarError: add_∇ not defined
Stacktrace:
 [1] getproperty(::Module, ::Symbol) at ./Base.jl:26
 [2] top-level scope at REPL[19]:1
 [3] include_string(::Function, ::Module, ::String, ::String) at ./loading.jl:1088

julia> Nabla.add_∇!
ERROR: UndefVarError: add_∇! not defined
Stacktrace:
 [1] getproperty(::Module, ::Symbol) at ./Base.jl:26
 [2] top-level scope at REPL[20]:1
 [3] include_string(::Function, ::Module, ::String, ::String) at ./loading.jl:1088

[oxinabox]

CI failures are unreleated