[link-quality] explicitly clear link quality for new child/router (#2941)

The child/router entries are generally cleared using `memset()` to avoid
explicitly setting all fields to zero.  As a result, it is necessary to
explicitly clear the link quality state to properly initialize the last
RSSI reading.  The user-specific constructors are also removed.

tests/unit/test_link_quality.cpp

@@ -93,6 +93,7 @@ void TestLinkQualityData(RssTestData aRssData)
     size_t          i;
 
     printf("- - - - - - - - - - - - - - - - - -\n");
+    linkInfo.Clear();
     min = kMinRssValue;
     max = kMaxRssValue;
 
@@ -387,6 +388,8 @@ void TestSuccessRateTracker(void)
 
     printf("\nTesting SuccessRateTracker\n");
 
+    rateTracker.Reset();
+
     VerifyOrQuit(rateTracker.GetSuccessRate() == kMaxRate, "SuccessRateTracker: Initial value incorrect");
     VerifyOrQuit(rateTracker.GetFailureRate() == 0, "SuccessRateTracker: Initial value incorrect");