Remove unused compute_api in ServerUsageController
Remove unused compute_api in ServerUsageController

compute_api is not used in ServerUsageController, then this patch
removes it for cleanup.

Change-Id: Idce514f63bebfc2b7f0e6df957ffaa06edd8be7a

cernops/nova,jianghuaw/nova,alaski/nova,vmturbo/nova,alaski/nova,sebrandon1/nova,bigswitch/nova,BeyondTheClouds/nova,cernops/nova,mahak/nova,CEG-FYP-OpenStack/scheduler,mikalstill/nova,vmturbo/nova,cyx1231st/nova,hanlind/nova,openstack/nova,cloudbase/nova,mikalstill/nova,mikalstill/nova,jianghuaw/nova,phenoxim/nova,openstack/nova,mahak/nova,cloudbase/nova,rahulunair/nova,klmitch/nova,rajalokan/nova,dims/nova,sebrandon1/nova,Juniper/nova,jianghuaw/nova,cernops/nova,Juniper/nova,gooddata/openstack-nova,hanlind/nova,hanlind/nova,BeyondTheClouds/nova,gooddata/openstack-nova,Juniper/nova,CEG-FYP-OpenStack/scheduler,rajalokan/nova,klmitch/nova,cyx1231st/nova,phenoxim/nova,zhimin711/nova,rahulunair/nova,sebrandon1/nova,dims/nova,cloudbase/nova,gooddata/openstack-nova,BeyondTheClouds/nova,vmturbo/nova,NeCTAR-RC/nova,jianghuaw/nova,bigswitch/nova,Juniper/nova,klmitch/nova,rajalokan/nova,NeCTAR-RC/nova,klmitch/nova,rajalokan/nova,gooddata/openstack-nova,rahulunair/nova,mahak/nova,vmturbo/nova,openstack/nova,zhimin711/nova