db/upnp/Discovery: eliminate attribute "last_seen"

src/db/upnp/Discovery.cxx

@@ -170,7 +170,7 @@ UPnPDeviceDirectory::expireDevices(Error &error)
 
 	for (auto it = directories.begin();
 	     it != directories.end();) {
-		if (now - it->second.last_seen > it->second.expires) {
+		if (now > it->second.expires) {
 			it = directories.erase(it);
 			didsomething = true;
 		} else {

src/db/upnp/Discovery.hxx

@@ -67,16 +67,14 @@ class UPnPDeviceDirectory {
 
 		/**
 		 * The MonotonicClockS() time stamp when this device
-		 * was last seen alive.
+		 * expires.
 		 */
-		unsigned last_seen;
-
-		unsigned expires; // seconds valid
+		unsigned expires;
 
 		ContentDirectoryDescriptor() = default;
 
 		ContentDirectoryDescriptor(unsigned last, int exp)
-			:last_seen(last), expires(exp+20) {}
+			:expires(last + exp + 20) {}
 
 		bool Parse(const std::string &url, const char *description,
 			   Error &_error) {