remove performance comment

Author: Miha Zgubic

docs/src/writing_good_rules.md

@@ -122,7 +122,7 @@ It is very easy to check gradients or derivatives with a computer algebra system
 In principle, a perfect AD system only needs rules for basic operations and can infer the rules for more complicated functions automatically.
 In practice, performance needs to be considered as well.
 
-Some functions use `ccall` internally for performance reasons, for example [`^`](https://github.com/JuliaLang/julia/blob/v1.5.3/base/math.jl#L886).
+Some functions use `ccall` internally, for example [`^`](https://github.com/JuliaLang/julia/blob/v1.5.3/base/math.jl#L886).
 These functions can not be differentiated through by AD systems, and need custom rules.
 
 Other functions can in principle be differentiated through by an AD system, but there exists a mathematical insight that can dramatically improve the computation of the derivative.