estimated_histogram: clean if0 and FIXME
estimated_histogram: clean if0 and FIXME

This patch cleans the estimated histogram implementation.
It removes the FIXME that were left in the code from the migration time
and the if0 commented out code.

Signed-off-by: Amnon Heiman <ed5fc74c4692051ad0b19749fd7cea6ec2d74b9e@scylladb.com>

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