Fix cleaning the indexes

Before that change, the cleaning of the channel indexes was wrong, because it
naively removed entire rows that was pertaining to the para we delete.
This approach is flawed because it doesn't account for the rows that are
pertaining to other paras that contain the outgoing one.

This clearly violates the invariant imposed on the indexes, that all
the index rows must contain alive paras, but apart from that it also
lead to the situation where ingress index would contain the a different
set of channels that an egress have.