Revert "x11: Filter out duplicate key presses when an IME is active"

This reverts commit f4813ca2cf.

src/video/x11/SDL_x11events.c

@@ -988,11 +988,8 @@ void X11_HandleKeyEvent(SDL_VideoDevice *_this, SDL_WindowData *windowdata, SDL_
     }
 
     if (pressed) {
-        // Duplicate events may be sent when an IME is active; don't send multiple keydown events for the same serial.
-        if (videodata->last_key_down_serial != xevent->xkey.serial) {
-            X11_HandleModifierKeys(videodata, scancode, true, true);
-            SDL_SendKeyboardKeyIgnoreModifiers(timestamp, keyboardID, keycode, scancode, true);
-        }
+        X11_HandleModifierKeys(videodata, scancode, true, true);
+        SDL_SendKeyboardKeyIgnoreModifiers(timestamp, keyboardID, keycode, scancode, true);
 
         // Synthesize a text event if the IME didn't consume a printable character
         if (*text && !(SDL_GetModState() & (SDL_KMOD_CTRL | SDL_KMOD_ALT))) {
@@ -1002,7 +999,6 @@ void X11_HandleKeyEvent(SDL_VideoDevice *_this, SDL_WindowData *windowdata, SDL_
         }
 
         X11_UpdateUserTime(windowdata, xevent->xkey.time);
-        videodata->last_key_down_serial = xevent->xkey.serial;
     } else {
         if (X11_KeyRepeat(display, xevent)) {
             // We're about to get a repeated key down, ignore the key up

src/video/x11/SDL_x11video.h

@@ -140,8 +140,6 @@ struct SDL_VideoData
     int xinput_master_pointer_device;
     bool xinput_hierarchy_changed;
 
-    unsigned long last_key_down_serial;
-
     int xrandr_event_base;
     struct
     {