player: remove higher-level remains of DVD/BD menu support

Nobody wanted to restore this, so it gets the boot.

If anyone still wants to volunteer to restore menu support, this would
be welcome. (I might even try it myself if I feel masochistic and like
wasting a lot of time for nothing.) But if it does get restored, it
should be done differently. There were many stupid things about how it
was done. For example, it somehow tried to pull mp_nav_events through
all the layers (including needing to "buffer" them in the demuxer),
which was needlessly complicated. It could be done simpler.

This code was already inactive, so this commit actually changes nothing.
Also keep in mind that normal DVD/BD playback still works.

input/cmd_list.c

@@ -167,8 +167,6 @@ const struct mp_cmd_def mp_cmds[] = {
   }},
   { MP_CMD_DISABLE_INPUT_SECTION, "disable-section", { ARG_STRING } },
 
-  { MP_CMD_DISCNAV, "discnav", { ARG_STRING } },
-
   { MP_CMD_AB_LOOP, "ab-loop", },
 
   { MP_CMD_DROP_BUFFERS, "drop-buffers", },
@@ -275,7 +273,6 @@ static const struct legacy_cmd legacy_cmds[] = {
     {"show_tracks",             "show-text ${track-list}"},
     {"show_playlist",           "show-text ${playlist}"},
     {"speed_mult",              "multiply speed"},
-    {"dvdnav",                  "discnav"},
 
     // Approximate (can fail if user added additional whitespace)
     {"pt_step 1",               "playlist-next"},