command: don't add SUB_SEEK_OFFSET twice with sub-seek

The name SD_CTRL_SUB_STEP is misleading, but it is also used for
sub-seek, and sub/sd_ass.c already adds SUB_SEEK_OFFSET with it.

player/command.c

@@ -5445,10 +5445,6 @@ static void cmd_sub_step_seek(void *p)
                     track_ind == 0 ? "sub-delay" : "secondary-sub-delay",
                     cmd->on_osd);
             } else {
-                // We can easily seek/step to the wrong subtitle line (because
-                // video frame PTS and sub PTS rarely match exactly). Add an
-                // arbitrary forward offset as a workaround.
-                a[0] += SUB_SEEK_OFFSET;
                 mark_seek(mpctx);
                 queue_seek(mpctx, MPSEEK_ABSOLUTE, a[0], MPSEEK_EXACT,
                            MPSEEK_FLAG_DELAY);