[Trivial]Remove unused _last_bw_usage_cell_update
[Trivial]Remove unused _last_bw_usage_cell_update

_last_bw_usage_cell_update is no used after 14cc25552a52e6898a81b9877c6d2c0423a95d57,
this is to remove it.

Change-Id: Ic249dbbb5a64808377d8127b0892c4ac8afaaa25

mahak/nova,klmitch/nova,klmitch/nova,openstack/nova,mahak/nova,rahulunair/nova,openstack/nova,rahulunair/nova,klmitch/nova,rahulunair/nova,mahak/nova,klmitch/nova,openstack/nova