Remove amqplib from global-requirements

amqplib is not used directly by OpenStack, but only via
kombu. Newer kombu versions use amqp by default, so
any dependency on amqplib is obsolete and can be removed.

Change-Id: I58da17d05cb29e320d8956c9d03a66722beedea1
Merge "Remove amqplib from global-requirements"