Trac #34682: clean up old gcc < 8 patches

One of these left after #34266 is

`build/pkgs/flint/patches/0001-flint.h-On-GCC-4.9-do-not-use-
_Thread_local.patch`

there are more (see the branch)

URL: https://trac.sagemath.org/34682
Reported by: dimpase
Ticket author(s): Dima Pasechnik
Reviewer(s): Matthias Koeppe

Author: Release Manager