tty.h: remove dead define

No longer used. TTY_FLIPBUF_SIZE will also go soon but needs a couple of
other cleanups first

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>