STCS_backlog: remove unused attribute
STCS_backlog: remove unused attribute

This attribute ended up being unused in the final version.
Spotted now while reading the code for other purposes.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla