Remove 'loopback address detection' helper function

It is not needed since `QHostAddress::isLoopback()` can do the job.
PR #21259.

src/base/utils/net.h

@@ -45,7 +45,6 @@ namespace Utils::Net
 
     bool isValidIP(const QString &ip);
     std::optional<Subnet> parseSubnet(const QString &subnetStr);
-    bool isLoopbackAddress(const QHostAddress &addr);
     bool isIPInSubnets(const QHostAddress &addr, const QList<Subnet> &subnets);
     QString subnetToString(const Subnet &subnet);
     QHostAddress canonicalIPv6Addr(const QHostAddress &addr);