[network-diagnostic] remove Max Child Timeout TLV from MTD build (#4730)

src/core/thread/mle_router_mtd.hpp

@@ -73,8 +73,6 @@ public:
     static bool IsRouterIdValid(uint8_t aRouterId) { return aRouterId <= kMaxRouterId; }
 
     otError SendChildUpdateRequest(void) { return Mle::SendChildUpdateRequest(); }
-
-    otError GetMaxChildTimeout(uint32_t &) { return OT_ERROR_NOT_IMPLEMENTED; }
 };
 
 } // namespace Mle

src/core/thread/network_diagnostic.cpp

@@ -467,6 +467,7 @@ otError NetworkDiagnostic::FillRequestedTlvs(Message &             aRequest,
             break;
         }
 
+#if OPENTHREAD_FTD
         case NetworkDiagnosticTlv::kMaxChildTimeout:
         {
             uint32_t maxTimeout = 0;
@@ -481,6 +482,7 @@ otError NetworkDiagnostic::FillRequestedTlvs(Message &             aRequest,
 
             break;
         }
+#endif
 
         default:
             ExitNow(error = OT_ERROR_PARSE);