#33 Remove unused built-in right-to-left text support

[William Kendrick]

Tux Paint's i18n.c has a now-empty look-up table lang_use_right_to_left_word which is used to fill wished_langs[].need_right_to_left_word. There is a note next to it:

/* FIXME: Remove! (We now require SDL_Pango all the time,
   so this is unnecessary -bjk 2023.04.30) */

FYI, a global exists in i18n.h that indicates how it was apparently used prior to that:

extern int need_right_to_left_word; // Words need to be reversed, too!
(e.g., Hebrew, but not Arabic)

That struct member is only used by textdir(), and since no language uses it, it appears textdir() itself can be removed, along with the rest of this stuff. It, in turn, is used by:
* do_render_button_label()
* do_prompt_image_flash_snd()
* do_open()
* do_slideshow()
* setup()