Rollup merge of #108164 - joboet:discard_messages_mpmc_array, r=Amanieu

Drop all messages in bounded channel when destroying the last receiver

Fixes #107466 by splitting the `disconnect` function for receivers/transmitters and dropping all messages in `disconnect_receivers` like the unbounded channel does. Since all receivers must be dropped before the channel is, the messages will already be discarded at that point, so the `Drop` implementation for the channel can be removed.

``@rustbot`` label +T-libs +A-concurrency

Author: matthiaskrgr