Bug 1754004 - Part 2: Remove the PFileDescriptorSet interface, r=asuth,necko-reviewers,kershaw

This interface should no longer be required due to the changes in part 1
limiting the complexity of IPCStream instances and limiting the number of file
descriptors which a single stream can attach to a message.

Removing this interface is necessary to serialize nsIInputStream instances over
arbitrary toplevel protocols and non-protocol IPC in the future.

Differential Revision: https://phabricator.services.mozilla.com/D141039