Delete unused down option from parse_docstring

[LilithHafner]

This is an NFC simplification of control flow.

After this merges, I plan to follow up with swapping the order of parse_public and parse_docstring and then adjusting the quote callsites of parse_eq to be parse_public. That should fix - JuliaLang/julia#51450 without allowing docstrings in quotes. (or, if we'd like, we can also allow docstrings in quotes)