test/lib/reader_lifecycle_policy: destroy_reader: cleanup context
test/lib/reader_lifecycle_policy: destroy_reader: cleanup context

Now that we don't rely on any external machinery to keep the relevant
parts of the context alive until needed as its life-cycle is effectively
enclosed in that of the life-cycle policy itself, we can cleanup the
context in `destroy_reader()` itself, avoiding a background trip back to
this shard.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla