Remove partition_pruning and drop_detached_partitions feature flags

What does this MR do and why?

Remove the feature flags for partition_pruning (#337153 (closed)) and drop_detached_partitions (#337155 (closed)).

These features are enabled successfully in production, and together dropped 2tb of data from the web_hook_logs table: #332199 (comment 733273043)

These flags are being removed in the same commit because the user-facing impact (reduced database size) isn't visible unless both are flipped. Changing them together gives a better changelog entry.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.