Now that all tables have their version stored in scylla_tables (ensured by ensure_committed_by_group0), the MD5 hash fallback in schema_mutations::digest() is dead code. Remove it along with feed_hash_for_schema_digest() and maybe_delete_schema_version(). Also include clenaups that became possible because of this code path removal.
No need to backport, code removal.
Closesscylladb/scylladb#30110
* https://github.com/scylladb/scylladb:
schema: remove register_feature_listeners
schema: remove reload_schema and schema recalculation support
schema: make in_memory_tables feature unconditional
schema: rename compact_for_schema_digest() to compact_for_comparison()
schema: remove legacy MD5-based schema version computation
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