Refactoring of qt_server.cc.

* Removed AA_EnableHighDpiScaling since it is already always on.

```
renderer/qt/qt_server.cc:110:34: warning: 'Qt::AA_EnableHighDpiScaling' is deprecated: High-DPI scaling is always enabled. This attribute no longer has any effect. [-Wdeprecated-declarations]
  110 |   QApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
      |                                  ^~~~~~~~~~~~~~~~~~~~~~~
```

#codehealth

PiperOrigin-RevId: 603732370

src/renderer/qt/BUILD.bazel

@@ -98,6 +98,7 @@ mozc_cc_qt_library(
         "//config:config_handler",
         "//ipc:named_event",
         "//protocol:renderer_cc_proto",
+        "@com_google_absl//absl/flags:flag",
     ],
 )
 

src/renderer/qt/qt_server.cc

@@ -34,17 +34,17 @@
 #include <QApplication>
 #include <QMetaType>
 #include <algorithm>
-#include <memory>
+#include <cstdint>
 #include <string>
-#include <utility>
 
+#include "absl/flags/flag.h"
 #include "base/logging.h"
 #include "base/system_util.h"
 #include "base/vlog.h"
-#include "client/client_interface.h"
 #include "config/config_handler.h"
 #include "ipc/named_event.h"
 #include "protocol/renderer_command.pb.h"
+#include "renderer/qt/qt_ipc_thread.h"
 
 // By default, mozc_renderer quits when user-input continues to be
 // idle for 10min.
@@ -75,7 +75,7 @@ QtServer::QtServer()
     : timeout_(0) {
   if (absl::GetFlag(FLAGS_restricted)) {
     absl::SetFlag(&FLAGS_timeout,
-                  // set 60sec with restricted mode
+                  // set 60 sec with restricted mode
                   std::min(absl::GetFlag(FLAGS_timeout), 60));
   }
 
@@ -106,10 +106,6 @@ void QtServer::Update(std::string command) {
 }
 
 int QtServer::StartServer(int argc, char **argv) {
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 6, 0))
-  QApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
-#endif  // QT_VERSION
-
   // |QWidget::move()| never works with wayland platform backend. Always use
   // 'xcb' platform backend.  https://github.com/google/mozc/issues/794
   ::setenv("QT_QPA_PLATFORM", "xcb", 1);