Remove the nova-manage flavor sub-command

The nova-manage flavor sub-command was deprecated in Juno milestone-1
via commit a601556684f18059e478ba645aff8276c9c78a4a with a note that it
will be removed in Kilo.

This removes it.

Change I4e2399f2fbab2c20299dd62c104b9220c1d36fb2 is to remove the
related Tempest tests.

Change-Id: I1798c8a0697009b3dcebeb818580c8323880e395
Merge "Remove the nova-manage flavor sub-command"