Revert "Use Linux 5.1 in build-many-glibcs.py."

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.

scripts/build-many-glibcs.py

@@ -714,7 +714,7 @@ class Context(object):
                             'gcc': 'vcs-9',
                             'glibc': 'vcs-mainline',
                             'gmp': '6.1.2',
-                            'linux': '5.1',
+                            'linux': '5.0',
                             'mpc': '1.1.0',
                             'mpfr': '4.0.2',
                             'mig': 'vcs-mainline',