[lld-macho][nfc] Simplify unwind section lookup

Previously, we stored a pointer from the ObjFile to its compact unwind
section in order to avoid iterating over the file's sections a second
time. However, given the small number of sections (not subsections) per
file, this caching was really quite unnecessary. We will soon do lookups
for more sections (such as the `__eh_frame` section), so let's simplify
the code first.

Reviewed By: #lld-macho, Roger

Differential Revision: https://reviews.llvm.org/D123434