Remove useless and non standard numext::atanh2 function.
Remove useless and non standard numext::atanh2 function.

Zefz/eigen,ritsu1228/eigen,Zefz/eigen,ritsu1228/eigen,ROCmSoftwarePlatform/hipeigen,ritsu1228/eigen,toastedcrumpets/eigen,TSC21/Eigen,TSC21/Eigen,pasuka/eigen,ROCmSoftwarePlatform/hipeigen,toastedcrumpets/eigen,pasuka/eigen,toastedcrumpets/eigen,pasuka/eigen,Zefz/eigen,ritsu1228/eigen,toastedcrumpets/eigen,ROCmSoftwarePlatform/hipeigen,Zefz/eigen,TSC21/Eigen,ROCmSoftwarePlatform/hipeigen,ritsu1228/eigen,pasuka/eigen,pasuka/eigen,TSC21/Eigen