Merge bitcoin/bitcoin#25466: ci: add unused-using-decls to clang-tidy

a02f3f19f5 tidy: use misc-unused-using-decls (fanquake)
d6787bc19b refactor: remove unused using directives (fanquake)
3617634324 validation: remove unused using directives (eugene)

Pull request description:

  Adds https://clang.llvm.org/extra/clang-tidy/checks/misc/unused-using-decls.html to our clang-tidy.
  PR'd after the discussion in #25433 (which it includes).

ACKs for top commit:
  jamesob:
    Github ACK a02f3f19f5

Tree-SHA512: 2bb937c1cc90006e69054458d845fb54f287567f4309c773a3fc859f260558c32ff51fc1c2ce9b43207426f3547e7ce226c87186103d741d5efcca19cd355253

src/qt/bitcoin.cpp

@@ -77,8 +77,6 @@ Q_DECLARE_METATYPE(CAmount)
 Q_DECLARE_METATYPE(SynchronizationState)
 Q_DECLARE_METATYPE(uint256)
 
-using node::NodeContext;
-
 static void RegisterMetaTypes()
 {
     // Register meta types used for QMetaObject::invokeMethod and Qt::QueuedConnection

src/qt/test/test_main.cpp

@@ -43,8 +43,6 @@ Q_IMPORT_PLUGIN(QAndroidPlatformIntegrationPlugin)
 #endif
 #endif
 
-using node::NodeContext;
-
 const std::function<void(const std::string&)> G_TEST_LOG_FUN{};
 
 const std::function<std::vector<const char*>()> G_TEST_COMMAND_LINE_ARGUMENTS{};