sys/ptrace.h: remove obsolete Linux PTRACE_SEIZE_DEVEL constant [BZ #21928]

Remove enum __ptrace_flags along with the only constant it contains,
PTRACE_SEIZE_DEVEL, from Linux's sys/ptrace.h files.

This temporary development constant shouldn't have been added to
sys/ptrace.h in the first place.  It was introduced in Linux by commit
v3.1-rc1~308^2~28 as a temporary part of new experimental PTRACE_SEIZE
interface.  Later, as PTRACE_SEIZE stabilized and lost its experimental
status, this flag was removed from Linux by commit v3.4-rc1~109^2~20.

* sysdeps/unix/sysv/linux/sys/ptrace.h (enum __ptrace_flags,
PTRACE_SEIZE_DEVEL): Remove.
* sysdeps/unix/sysv/linux/aarch64/sys/ptrace.h: Likewise.
* sysdeps/unix/sysv/linux/ia64/sys/ptrace.h: Likewise.
* sysdeps/unix/sysv/linux/powerpc/sys/ptrace.h: Likewise.
* sysdeps/unix/sysv/linux/s390/sys/ptrace.h: Likewise.
* sysdeps/unix/sysv/linux/sparc/sys/ptrace.h: Likewise.