Remove "build" from print_usage()

cmd_build was removed in I3c6b8a59b4935cd46d38639ad369fe21f35f082e,
but the corresponding entry in print_usage() has lingered. Remove
"build" from that function so as not to confuse users.

https://wiki.openstack.org/wiki/Trove/dev-env should probably
be edited to remove the "build" step as well.

Change-Id: I2b7c39b565ce4147ec03adaa583881ba936e4616