net: ipc_router: Remove registered server when an endpoint exits

When an endpoint gets released, remove the server registered with that
endpoint. This will reduce the number of messages being exchanged when
an endpoint hosting a server exits.

Change-Id: Idd539f92b2d11e57eec81265964c465b0782d406
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>