Clean up the plat and ui API

Remove functions no longer defined or used on any plat/UI
Remove the old non-multi-monitor-aware plat_resize() and rename
plat_resize_monitor() to plat_resize()

src/qt/qt_platform.cpp

@@ -149,6 +149,11 @@ strnicmp(const char *s1, const char *s2, size_t n)
 #endif
 }
 
+void
+do_start(void)
+{
+}
+
 void
 do_stop(void)
 {
@@ -425,12 +430,6 @@ plat_power_off(void)
     QTimer::singleShot(0, (const QWidget *) main_window, &QMainWindow::close);
 }
 
-void
-set_language(uint32_t id)
-{
-    lang_id = id;
-}
-
 extern "C++" {
 QMap<uint32_t, QPair<QString, QString>> ProgSettings::lcid_langcode = {
     { 0x0403, { "ca-ES", "Catalan (Spain)" }         },
@@ -741,12 +740,6 @@ plat_get_cpu_string(char *outbuf, uint8_t len) {
 
 }
 
-double
-plat_get_dpi(void)
-{
-    return util::screenOfWidget(main_window)->devicePixelRatio();
-}
-
 void
 plat_set_thread_name(void *thread, const char *name)
 {