Don't remove entries from an existing scancode keymap

If we can't find the X11 keysym, it's likely that either the keysym is NoSymbol, in which case we won't hit it anyway, or it's been mapped to a character, in which case the existing mapping is correct for the scancode and the character will be reflected in the keycode mapping.

src/video/x11/SDL_x11keyboard.c

@@ -655,7 +655,7 @@ X11_InitKeyboard(_THIS)
             if (scancode == data->key_layout[i]) {
                 continue;
             }
-            if (scancode == SDL_SCANCODE_UNKNOWN || default_keymap[scancode] >= SDLK_SCANCODE_MASK) {
+            if (default_keymap[scancode] >= SDLK_SCANCODE_MASK) {
                 /* Not a character key, safe to remap */
 #ifdef DEBUG_KEYBOARD
                 SDL_Log("Changing scancode, was %d (%s), now %d (%s)\n", data->key_layout[i], SDL_GetScancodeName(data->key_layout[i]), scancode, SDL_GetScancodeName(scancode));