sampling: StatsThread: dropped unused methods

This is only a first step twards removal of the useless base class
StatsThread.

Change-Id: Id92031ea787aa41a97f7a1ab488065b06f29673e
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/20672
Tested-by: Petr Šebek <psebek@redhat.com>
Reviewed-by: Petr Šebek <psebek@redhat.com>