Revert "Introduce AMQ::Client::EventMachineClient::Deferrable#append_callback(&block)"
Revert "Introduce AMQ::Client::EventMachineClient::Deferrable#append_callback(&block)"

This reverts commit 64fba4ab42077cf85bbc1b10915838debf442533.

Actually, current EM::Deferrable behavior is FIFO (it unshifts new callbacks but
also uses Array#pop to iterate over them). I am an idiot.

travis-repos/amq-client,travis-repos/amq-client,ruby-amqp/amq-client,ruby-amqp/amq-client