database: remove cpu_flush_quota metric
database: remove cpu_flush_quota metric

We can now grab that from the CPU scheduler, that exports both runtime
and shares.

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

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