Use C++17's [[nodiscard]] instead of WARN_UNUSED_RESULT

No functionality change. Also remove some header guards along the way.

Test: m
Change-Id: I1afdcaea95a3dd56f392c4e61d7670f43615792a
diff --git a/server/NetworkController.cpp b/server/NetworkController.cpp
index 1e4894f..2c77830 100644
--- a/server/NetworkController.cpp
+++ b/server/NetworkController.cpp
@@ -47,8 +47,7 @@
 
 using android::netdutils::DumpWriter;
 
-namespace android {
-namespace net {
+namespace android::net {
 
 namespace {
 
@@ -67,21 +66,21 @@
 // setPermissionForNetworks).
 // TODO: use std::mutex and GUARDED_BY instead of manual inspection.
 class NetworkController::DelegateImpl : public PhysicalNetwork::Delegate {
-public:
+  public:
     explicit DelegateImpl(NetworkController* networkController);
     virtual ~DelegateImpl();
 
-    int modifyFallthrough(unsigned vpnNetId, const std::string& physicalInterface,
-                          Permission permission, bool add) WARN_UNUSED_RESULT;
+    [[nodiscard]] int modifyFallthrough(unsigned vpnNetId, const std::string& physicalInterface,
+                                        Permission permission, bool add);
 
-private:
-    int addFallthrough(const std::string& physicalInterface,
-                       Permission permission) override WARN_UNUSED_RESULT;
-    int removeFallthrough(const std::string& physicalInterface,
-                          Permission permission) override WARN_UNUSED_RESULT;
+  private:
+    [[nodiscard]] int addFallthrough(const std::string& physicalInterface,
+                                     Permission permission) override;
+    [[nodiscard]] int removeFallthrough(const std::string& physicalInterface,
+                                        Permission permission) override;
 
-    int modifyFallthrough(const std::string& physicalInterface, Permission permission,
-                          bool add) WARN_UNUSED_RESULT;
+    [[nodiscard]] int modifyFallthrough(const std::string& physicalInterface, Permission permission,
+                                        bool add);
 
     NetworkController* const mNetworkController;
 };
@@ -850,5 +849,4 @@
     }
 }
 
-}  // namespace net
-}  // namespace android
+}  // namespace android::net