cache_flat_mutation_reader: Remove raiish comparator
cache_flat_mutation_reader: Remove raiish comparator

The instance of position_in_partition::tri_compare sits
on the reader itself and just occupies memory. It can be
created on demand all the more so it's only one place that
needs it.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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