[locator] remove OwnerLocator class (#6138)

This commit removes the OwnerLocator class since it's actually
useless.

Timer and Tasklet inherited from OwnerLocator to so that they can use
GetOwner<Type>() to find the owner object. However, these two classes
already inherited from InstanceLocator, thus can use Get<Type>() to
get any component of the OT instance.

tests/unit/test_timer.cpp

@@ -87,7 +87,7 @@ template <typename TimerType> class TestTimer : public TimerType
 {
 public:
     explicit TestTimer(ot::Instance &aInstance)
-        : TimerType(aInstance, TestTimer::HandleTimerFired, nullptr)
+        : TimerType(aInstance, TestTimer::HandleTimerFired)
         , mFiredCounter(0)
     {
     }