Summary:
OpsStream is retired (SR tier drained, SEV S505893) and all fbcode/www clients have landed. This removes the thrift interface `fbcode/opsstream/if` (`//opsstream/if:if-opsstream` + `opsstream-asyncio`) and syncs the deletion into www via `ThriftPkgInfo remove` + `meerkat`: drops the generated Hack client, thrift mirror, package info, the two prod `overrides.json` entries, and the now-dangling `OpsStreamEvent` from `ThriftUtil::INTISH_CAST_STRUCTS`.

`configerator-hook/` (config_infra-owned fbpkg with external cfgate registration) and the top-level `PACKAGE` are intentionally left for a separate config_infra follow-up.

Differential Revision: D112524544

fbshipit-source-id: 49812567221ba9535916e304f38b764fa4d29a35