net: ipc_router: Remove unused header file

subsystem_notif.h is not used to receive any notifications regarding
subsystem events. The underlying links are used to receive such events.
Hence remove including the unused header file.

Change-Id: I2b87ec34079f96e98082790b4ba7a0a3ad6ef626
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>