migration_manager: Call maybe_sync from this
migration_manager: Call maybe_sync from this

The only caller of maybe_sync() method is now the method itself
and can stop using global migration manager instance and switch
to using 'this'.

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

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