Remove instance_info_cache_delete() from conductor

This was not used in v2.0 of the RPC API, so we can remove it
outright without deprecating it in conductor manager.

Related to blueprint compute-manager-objects-juno

Change-Id: I68068c1d0ef8f346406fe3ccb341a8d0a2474bdd
Merge "Remove instance_info_cache_delete() from conductor"