um: remove unused reactivate_chan() declaration

All uses of reactivate_chan() were removed by
commit 940b241d ("um: Remove obsolete reenable_XX
calls"), so remove the declaration, too.
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/chan.h

@@ -36,7 +36,6 @@ extern int console_write_chan(struct chan *chan, const char *buf,
 			      int len);
 extern int console_open_chan(struct line *line, struct console *co);
 extern void deactivate_chan(struct chan *chan, int irq);
-extern void reactivate_chan(struct chan *chan, int irq);
 extern void chan_enable_winch(struct chan *chan, struct tty_port *port);
 extern int enable_chan(struct line *line);
 extern void close_chan(struct line *line);