[libcxx][docs] Remove completed issues from TODO.TXT

Remove tasks listed that refer to papers or issues that are marked complete in
`libcxx/docs/Status/Cxx17Papers.csv` or `libcxx/docs/Status/Cxx17Issues.csv`

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D108856

GitOrigin-RevId: 2498f8fd76c2376b77f5c493da8873f049394916
1 file changed