Remove unused flag 'host_state_interval'

Patch I749204d7d6461993d0c42bf5877b65c5ec17c62f removed periodic task
`_report_driver_status()`, which fanout this host's capabilities to
nova scheduler. So the flag 'host_state_interval' is not used.
Just remove it in this patch.

DocImpact: the 'host_state_interval' flag has been removed. It was last used in
grizzly and was not used in havana.

Change-Id: Ieb047ccb33cdecc6f25b6ae128dcac4e1c00cd98
Merge "Remove unused flag 'host_state_interval'"