Use keys with hash tag in manifest import metadata

What does this MR do and why?

This MR releases the feature to use keys with hash tag in manifest import metadata. It also cleans up the manifest_import_use_hash_tagged_key feature flag.

The feature flag was introduced in !124871 (merged).

This is the 2nd part of #416350 (closed) (feature flag removal/"feature" release).

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Before	After

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