Fix ChainRulesCore extension

Author: oschulz

ext/MeasureBaseChainRulesCoreExt.jl

@@ -42,9 +42,6 @@ ChainRulesCore.rrule(::typeof(check_dof), ν, μ) = check_dof(ν, μ), _check_do
 _checked_arg_pullback(ΔΩ) = NoTangent(), NoTangent(), ΔΩ
 ChainRulesCore.rrule(::typeof(checked_arg), ν, x) = checked_arg(ν, x), _checked_arg_pullback
 
-_check_dof_pullback(ΔΩ) = NoTangent(), NoTangent(), NoTangent()
-ChainRulesCore.rrule(::typeof(check_dof), ν, μ) = check_dof(ν, μ), _check_dof_pullback
-
 # = return type inference ====================================================
 
 using MeasureBase: logdensityof_rt