During automatic recovery, in https://github.com/ruby-amqp/march_hare/blob/master/lib/march_hare/session.rb#L260 a new thread pool is created and saved to @thread_pool, but nowhere else is that variable ever reused.
Remove unused thread_pool
When in commit ea3c5ef march_hare's thread pools were deprecated in
favor of RMQ's thread pools, one thread pool was left behind, but it
is/was no longer used, and can be safely removed.