Remove unused db.api.aggregate_host_get_by_metadata_key

The last use was removed in change
I2ec7373f063ed728dde2af3b7c8259f4391885bc:

object-ify availability_zones

Change-Id: Ie3dd4a3b3008af377a46ccc4dcadb6409101bc2d
Merge "Remove unused db.api.aggregate_host_get_by_metadata_key"