[lld/mac] Delete incorrect FIXME

"""Bitcode symbols only exist before LTO runs, and only serve the purpose of
resolving visibility so LTO can better optimize. Running LTO creates ObjFiles
from BitcodeFiles, and those ObjFiles contain regular Defined symbols (with
isec set and all) that will replace the bitcode symbols. So things should
(hopefully) work as-is :)"""

  -- https://reviews.llvm.org/rGdbbc8d8333f29cf4ad6f4793da1adf71bbfdac69#inline-6081