mISDN: Remove obsolete PIPELINE_DEBUG debugging information

As Leon Romanovsky's tips:
The definition of macro PIPELINE_DEBUG is commented more than 10 years ago
and can be seen as a dead code that should be removed.

Suggested-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>