Forget everything you learned in college...
"Forget everything you learned in college. You won't need it working here."
This is a joke that makes fun of the requirements that recruiters make for performing certain jobs. If you don't need to have specific knowledge in order to perform the duties of a job, then why require them in order to obtain the job?
"But I never went to college."
"Well then, I'm sorry. You are underqualified to work here."
The purpose of this joke is perhaps more social commentary than humor, however, it is sort of funny in the "sad but true" way. I suppose that it is a form of irony.