options: remove --volstep

It's just confusing; users are encouraged to edit input.conf instead
(changing the argument to the "add" command).

Update input.conf to keep the old behavior.

options/m_config.c

@@ -981,5 +981,6 @@ static const char *const replaced_opts =
     "|forceidx#--index"
     "|cache-pause-below#for 'no', use --no-cache-pause"
     "|no-cache-pause-below#--no-cache-pause"
+    "|volstep#edit input.conf directly instead"
     "|"
 ;

options/options.c

@@ -360,7 +360,6 @@ const m_option_t mp_opts[] = {
                 {"yes", SOFTVOL_YES},
                 {"auto", SOFTVOL_AUTO})),
     OPT_FLOATRANGE("softvol-max", softvol_max, 0, 10, 10000),
-    OPT_INTRANGE("volstep", volstep, 0, 0, 100),
     OPT_FLOATRANGE("volume", mixer_init_volume, 0, -1, 100),
     OPT_CHOICE("mute", mixer_init_mute, M_OPT_OPTIONAL_PARAM,
                ({"auto", -1},
@@ -561,7 +560,6 @@ const struct MPOpts mp_default_opts = {
     .softvol_max = 200,
     .mixer_init_volume = -1,
     .mixer_init_mute = -1,
-    .volstep = 2,
     .gapless_audio = -1,
     .audio_buffer = 0.2,
     .vo = {

options/options.h

@@ -71,7 +71,6 @@ typedef struct MPOpts {
     float mixer_init_volume;
     int mixer_init_mute;
     char *mixer_restore_volume_data;
-    int volstep;
     float softvol_max;
     int gapless_audio;
     double audio_buffer;