
It could be that:
Rafiki is annoying Simba to get his attention
Simba tells him to cut it out
But Rafiki needs Simba's attention so he continues
He can't 'cut out' being annoying because he needs to speak to Simba, and if he did cut it out, he still needs to speak to Simba, so he would annoy him again, effectivly his annoyance 'growing back'
Sorry if that makes little sense, it's incredibly difficult to explain what i'm thinking

As well as I, I never looked into deeper meaning of when Rafiki says he can't cut it out because it'll grow right back. But, as I reading the comments on this forum, I agree more with Simbaholic. I doubt Rafiki is referring to his fur (though it will grow if you cut it), but to his annoying of Simba.
To paraphase Simbaholic's word, "cut it out" means to stop, and Simba wants Rafiki to stop annoying him. If Rafiki stops annoying Simba for a minute (which is when he "cuts it out"), he'll continue annoying him later on that night, therefore that's how the annoyance "grows back", so he can get Simba's awareness on revisiting Mufasa.