Removing a feeding tube, when consistent with the patient's wishes, is not cruel; rather, when appropriate, it is one of the most humane ways to permit a dead or dying brain's body to die.
Coincidentally at the same time, I had to have my cat put to sleep. The vet came to my house, he received a single injection, and then he peacefully fell asleep in my arms without so much as a twitch. The entire process took not five minutes altogether. I know which way of dying I'd prefer, if it was my terminally ill body at stake.