[bug] cleaning up prerequisites

[knightron0]

We're getting the prerequisites from SelfService (example CS251) and then parsing them into CNF/DNF form through RegEx. However, neither scraping nor parsing accuracy is 100%. A few issues that we've faced:

1. Outdated courses that we can't find anymore (see all unlinked courses here)
2. Repeated courses on the same level of logic (example MA165 here)
3. The prerequisite format differs on the catalog link so our scraping fails. This usually happens with grad courses (see CS523) but also can happen with others like CS182.