It is used only on hurd.
No semantic change is expected, checked on a build against x86_64-linux-gnu,
alpha-linux-gnu, mips64-linux-gnu, and sparc64-linux-gnu.
* sysdeps/unix/sysv/linux/bits/termios.h (_IOT_termios): Remove.
* sysdeps/unix/sysv/linux/alpha/bits/termios.h: Likewise.
* sysdeps/unix/sysv/linux/mips/bits/termios.h: Likewise.