utils/extremum_tracking: drop default constructor
utils/extremum_tracking: drop default constructor

Default constructed extremum_tracker has uninitialised _default_value
which basically makes it never correct to do that. Since this class is a
mechanism and not a value it doesn't really need to be a regular type,
so let's drop the default constructor.

Message-Id: <d4735742e7a9e36534efe18e3dd61c2886892387@scylladb.com>

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