Remove instance_metadata_get_all* from db api

The following calls are no longer called by nova.compute.api and should
be removed along with their associated tests:

db.api.instance_metadata_get_all
db.api.sqlalchemy.instance_metadata_get_all
db.api.sqlalchemy._instance_metadata_get_all_query

Change-Id: Id02786127ee35124e58425c699e5d3e84f42f527