skeleton_reader: inherit from flat_mutation_reader_v2::impl
skeleton_reader: inherit from flat_mutation_reader_v2::impl

(completely mechanical change)

Signed-off-by: Michael Livshin <ae60993c65de9cdf2ca07b7426c976808535d555@scylladb.com>
Message-Id: <20220406122912.2248111-1-ae60993c65de9cdf2ca07b7426c976808535d555@scylladb.com>

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