options: remove broken --video-stereo-mode option

See changelog for minor explanation. Basically, 3D is unused crap and
nobody cares.

DOCS/interface-changes.rst

@@ -94,6 +94,10 @@ Interface changes
       the future. (This kind of waiting was always a feature to prevent that
       playback is started while scripts are only half-loaded.)
     - deprecate --ovoffset, --oaoffset, --ovfirst, --oafirst
+    - remove --video-stereo-mode. This option was broken out of laziness, and
+      nobody wants to fix it. Automatic 3D down-conversion to 2D is also broken,
+      although you can just insert the stereo3d filter manually. The obscurity
+      of 3D content doesn't justify such an option anyway.
  --- mpv 0.28.0 ---
     - rename --hwdec=mediacodec option to mediacodec-copy, to reflect
       conventions followed by other hardware video decoding APIs

options/options.c

@@ -583,9 +583,6 @@ const m_option_t mp_opts[] = {
     OPT_STRING("force-media-title", media_title, 0),
     OPT_CHOICE_OR_INT("video-rotate", video_rotate, UPDATE_IMGPAR, 0, 359,
                       ({"no", -1})),
-    OPT_CHOICE_C("video-stereo-mode", video_stereo_mode, UPDATE_IMGPAR,
-                 mp_stereo3d_names,
-                 .deprecation_message = "mostly broken"),
 
     OPT_CHOICE_OR_INT("cursor-autohide", cursor_autohide_delay, 0,
                       0, 30000, ({"no", -1}, {"always", -2})),

options/options.h

@@ -174,7 +174,6 @@ typedef struct MPOpts {
     int cursor_autohide_fs;
 
     int video_rotate;
-    int video_stereo_mode;
 
     char *audio_decoders;
     char *video_decoders;