[mdns] allow UnsubscribeService/Host() to remove non-present entries (

openthread#2206)

This commit updates `UnsubscribeService/Host()` to allow them to be
called with service or host name that may not be present in the
list. This situation may happen if duplicate queries are received
for the same names.

src/mdns/mdns_mdnssd.cpp

@@ -948,7 +948,7 @@ void PublisherMDnsSd::UnsubscribeService(const std::string &aType, const std::st
                       [&aType, &aInstanceName](const std::unique_ptr<ServiceSubscription> &aService) {
                           return aService->mType == aType && aService->mInstanceName == aInstanceName;
                       });
-    assert(it != mSubscribedServices.end());
+    VerifyOrExit(it != mSubscribedServices.end());
 
     mSubscribedServices.erase(it);
 
@@ -998,7 +998,7 @@ void PublisherMDnsSd::UnsubscribeHost(const std::string &aHostName)
         mSubscribedHosts.begin(), mSubscribedHosts.end(),
         [&aHostName](const std::unique_ptr<HostSubscription> &aHost) { return aHost->mHostName == aHostName; });
 
-    assert(it != mSubscribedHosts.end());
+    VerifyOrExit(it != mSubscribedHosts.end());
 
     mSubscribedHosts.erase(it);