Removed kombu from requirements

Since we've replaced oslo-incubator RPC layer with oslo.messaging, we
don't ship any code that uses kombu.

Change-Id: Ia8a74f1326ecd98c47cbe447f04d475bf61e19d3
(cherry picked from commit 424c7faa75d96950d80f49f20f5414d1a297af72)
Merge "Removed kombu from requirements" into proposed/juno