tty: Drop TTY_SUBTYPE_SLAVE from /dev/console and /dev/tty

They are confusing code readers. Lets save these @subtypes
for Unix98 pty peers where they are really important.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>