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>