Just put your hook in /etc/pacman.d/hooks and uncomment the line
#HookDir = /etc/pacman.d/hooks/
in
/etc/pacman.conf
I would delete:
Operation = remove
There will be no grub-install to talk about if the package Grub is removed.
Operation = install
seems unnecessary as well, since the package is already installed.
The hook needs to be triggered only when the package is upgraded.