Revert "recommend using rule definition tools"

This reverts commit cca44e7.

docs/src/writing_good_rules.md

@@ -67,40 +67,6 @@ julia> rrule(foo, 2)
 While this is more verbose, it ensures that if an error is thrown during the `pullback` the [`gensym`](https://docs.julialang.org/en/v1/base/base/#Base.gensym) name of the local function will include the name you gave it.
 This makes it a lot simpler to debug from the stacktrace.
 
-## Use rule definition tools
-
-Rule definition tools can help you write more `frule`s and the `rrule`s with less lines of code.
-
-### [`@non_differentiable`](@ref)
-
-For non-differentiable functions the [`@non_differentiable`](@ref) macro can be used.
-For example, instead of manually defining the `frule` and the `rrule` for string concatenation `*(String..)`, the macro call
-```
-@non_differentiable *(String...)
-```
-defines the following `frule` and `rrule` automatically
-```
-function ChainRulesCore.frule(var"##_#1600", ::Core.Typeof(*), String::Any...; kwargs...)
- return (*(String...; kwargs...), DoesNotExist())
-end
-function ChainRulesCore.rrule(::Core.Typeof(*), String::Any...; kwargs...)
- return (*(String...; kwargs...), function var"*_pullback"(_)
- (Zero(), ntuple((_->DoesNotExist()), 0 + length(String))...)
- end)
-end
-```
-Note that the types of arguments are propagated to the `frule` and `rrule` definitions.
-This is needed in case the function differentiable for some but not for other types of arguments.
-For example `*(1, 2, 3)` is differentiable, and is not defined with the macro call above.
-
-### [`@scalar_rule`](@ref)
-
-For functions involving only scalars, i.e. subtypes of `Number` (no `struct`s, `String`s...), both the `frule` and the `rrule` can be defined using a single [`@scalar_rule`](@ref) macro call. 
-
-Note that the function does not have to be $\mathbb{R} \rightarrow \mathbb{R}$.
-In fact, any number of scalar arguments is supported, as is returning a tuple of scalars.
-
-See docstrings for the comprehensive usage instructions.
 ## Write tests
 
 In [ChainRulesTestUtils.jl](https://github.com/JuliaDiff/ChainRulesTestUtils.jl)