build: make libass non-optional

Using mpv without libass isn't really supported, since it's not only
used to display ASS subtitles, but all text subtitles, and even OSD.

At least 1 user complained that the player printed a warning if built
without libass. Avoid trying to create the impression that using this
software without libass is in any way supported or desirable, and make
it fully mandatory.

(As far as making dependencies optional goes, I'd rather make ffmpeg
optional, which is an oversized and bloated library, rather than
something tiny like libass.)

sub/dec_sub.c

@@ -39,9 +39,7 @@ extern const struct sd_functions sd_lavc;
 
 static const struct sd_functions *const sd_list[] = {
     &sd_lavc,
-#if HAVE_LIBASS
     &sd_ass,
-#endif
     NULL
 };