maint: remove unused exceptions
maint: remove unused exceptions

* nova/exception (InstanceSystemMetadataNotFound):
Unreferenced since commit 0d417fa73
* nova/exception (CellError): Never referenced.

Change-Id: I30449748fe1156adf7da03aaace565e63bdac144

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects