[build] fix GCC7 warnings (#2090)

src/core/coap/coap.cpp

@@ -616,8 +616,7 @@ void Coap::ProcessReceivedResponse(Header &aResponseHeader, Message &aMessage,
         // Send empty ACK if it is a CON message.
         SendAck(aResponseHeader, aMessageInfo);
 
-        // fall through
-        ;
+    // fall through
 
     case OT_COAP_TYPE_NON_CONFIRMABLE:
         // Separate response.

src/core/thread/link_quality.cpp

@@ -243,17 +243,17 @@ uint8_t LinkQualityInfo::CalculateLinkQuality(uint8_t aLinkMargin, uint8_t aLast
     case 0:
         threshold1 += kHysteresisThreshold;
 
-    // Intentional fall-through to next case.
+    // fall-through
 
     case 1:
         threshold2 += kHysteresisThreshold;
 
-    // Intentional fall-through to next case.
+    // fall-through
 
     case 2:
         threshold3 += kHysteresisThreshold;
 
-    // Intentional fall-through to next case.
+    // fall-through
 
     default:
         break;

src/core/thread/mle_router.cpp

@@ -4588,7 +4588,7 @@ void MleRouter::RemoveChildren(void)
         case Neighbor::kStateValid:
             GetNetif().SetStateChangedFlags(OT_CHANGED_THREAD_CHILD_REMOVED);
 
-        // Fall-through to next case
+        // fall-through
 
         case Neighbor::kStateChildUpdateRequest:
         case Neighbor::kStateRestored: