Remove inaccurate mapping of 'host-meta' in csv

Now we don't have a similar command to 'nova host-meta' in osc,
the 'host set/unset' in osc is used to manage host properties.
Need to remove the inaccurate mapping in nova.csv.

Change-Id: I291174a29c8e3419b62d47264dedf176262a816c
Closes-Bug: #1690902

Author: wingwj

doc/source/cli/data/nova.csv

@@ -47,7 +47,7 @@ host-describe,host show,Describe a specific host.
 host-evacuate,,Evacuate all instances from failed host.
 host-evacuate-live,,Live migrate all instances of the specified host to other available hosts.
 host-list,host list,List all hosts by service.
-host-meta,host set / unset,Set or Delete metadata on all instances of a host.
+host-meta,,Set or Delete metadata on all instances of a host.
 host-servers-migrate,,Cold migrate all instances off the specified host to other available hosts.
 host-update,host set,Update host settings.
 hypervisor-list,hypervisor list,List hypervisors.