Remove metadata deletion endpoints and algorithms

This was expected to be used in these two cases:

1. if we remove mappings or file detection from a metadata indexer
2. if an origin removes all its metadata files

but:

1. if we do so, then we should bump the indexer version, so the old metadata will be preserved anyway, as different indexer versions get different indexer_configuration_ids
2. this should be a rather rare even, and even if it happens, we might want to keep the old metadata anyway rather than nothing (even if it's outdated), for search purposes.

Additionally, this commit is motivated by:

• that's less issues to deal with when writing to Kafka (the journal writer currently doesn't support suppression; and we would also have to add support for deletion in all consumers)
• less code (~250 lines)