[style] remove redundant control flow (#3705)

src/core/coap/coap.cpp

@@ -661,8 +661,6 @@ exit:
             cachedResponse->Free();
         }
     }
-
-    return;
 }
 
 CoapMetadata::CoapMetadata(bool                    aConfirmable,

src/core/net/dns_client.cpp

@@ -508,8 +508,6 @@ exit:
     {
         FinalizeDnsTransaction(*message, queryMetadata, NULL, 0, error);
     }
-
-    return;
 }
 
 } // namespace Dns

src/core/net/sntp_client.cpp

@@ -384,8 +384,6 @@ exit:
     {
         FinalizeSntpTransaction(*message, queryMetadata, 0, error);
     }
-
-    return;
 }
 
 } // namespace Sntp

src/core/thread/address_resolver.cpp

@@ -580,8 +580,6 @@ exit:
     {
         otLogWarnArp("Error while processing address error notification: %s", otThreadErrorToString(error));
     }
-
-    return;
 }
 
 void AddressResolver::HandleAddressQuery(void *aContext, otMessage *aMessage, const otMessageInfo *aMessageInfo)

src/core/thread/mle_router.cpp

@@ -4012,8 +4012,6 @@ exit:
 
     // send announce after received address solicit reply if needed
     InformPreviousChannel();
-
-    return;
 }
 
 void MleRouter::HandleAddressSolicit(void *aContext, otMessage *aMessage, const otMessageInfo *aMessageInfo)

src/core/thread/network_data_leader_ftd.cpp

@@ -284,8 +284,6 @@ exit:
     {
         SendCommissioningSetResponse(aMessage, aMessageInfo, state);
     }
-
-    return;
 }
 
 void Leader::HandleCommissioningGet(void *aContext, otMessage *aMessage, const otMessageInfo *aMessageInfo)

src/ncp/ncp_base.cpp

@@ -824,7 +824,6 @@ void NcpBase::UpdateChangedProps(void)
 
 exit:
     mDidInitialUpdates = true;
-    return;
 }
 
 // ----------------------------------------------------------------------------