It's Anthony Fantano coming to terms with his split personality, he decided to "kill" his other self in an attempt at curing himself and moving on.
You could graduatelly see Anthony lash out more and more towards his other self and since never took any offence or never really did what Anthony asked him to, there wasn't a single thing Anthony could do to get rid of Cal.
The last year or so the conversations with Cal had gotten to a minimum, mostly Cal interuping Anthony for a few seconds, you can clearly see Anthony tries to pretend Cal isn't there.
It's a possibility that Anthony started taking medication this year and Cal slowly lost his importance to Anthony, he didn't need something to project his insecurities on anymore.
Cal Chuchesta isn't very smart, he speaks with a slang and has terrible taste in music and ofcourse the iconic mustache, these are all things Anthony is scared of having or being.
Now Anthony can finally move on and live a normal life, it's what he always wanted.