This reverts commit c2b11710fb. Linux 5.1 headers are not in fact usable for glibc testing, because "[PATCH] uapi: avoid namespace conflict in linux/posix_types.h" <https://lore.kernel.org/lkml/20190319165123.3967889-1-arnd@arndb.de/> did not get merged for 5.1 and so many conform/ tests fail.
c2b11710fb