The API /storage_service/truncate/{ks} returns an unimplemented
error when invoked. As we already have a CQL command,
`TRUNCATE TABLE ks.cf` that causes the table to be truncated on all
nodes, the API can be dropped. Due to the error, it is unused.
Fixes https://github.com/scylladb/scylladb/issues/10520
No backport is required. A small cleanup of not working API.
Closesscylladb/scylladb#22258