osd/scheduler: rely on copy ellision to move return val

osd/scheduler: rely on copy ellision to move return val

C++14 enforces copy ellision in this case.

also silences warning of

src/osd/scheduler/mClockScheduler.cc:122:21: warning: redundant move in return statement [-Wredundant-move]
  122 |     return std::move(ret);
      |            ~~~~~~~~~^~~~~

Signed-off-by: Kefu Chai <kchai@redhat.com>