drivers/net/wan/x25_asy: Remove an unused flag "SLF_OUTWAIT"

The "SLF_OUTWAIT" flag defined in x25_asy.h is not actually used.
It is only cleared at one place in x25_asy.c but is never read or set.
So we can remove it.

Signed-off-by: Xie He <xie.he.0141@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>