Do not store subtitle-offset as gsetting

This is not something we should remember

data/com.github.rafostar.Clapper.gschema.xml

@@ -52,10 +52,6 @@
     </key>
 
     <!-- Subtitles -->
-    <key name="subtitle-offset" type="i">
-      <default>0</default>
-      <summary>Offset time for subtitle tracks relative to video (milliseconds)</summary>
-    </key>
     <key name="subtitle-font" type="s">
       <default>"Sans 12"</default>
       <summary>The subtitles font description</summary>

src/player.js

@@ -678,11 +678,6 @@ class ClapperPlayer extends GstClapper.Clapper
                 this.set_audio_video_offset(value);
                 debug(`set audio-video offset: ${value}`);
                 break;
-            case 'subtitle-offset':
-                value = Math.round(settings.get_int(key) * -Gst.MSECOND);
-                this.set_subtitle_video_offset(value);
-                debug(`set subtitle-video offset: ${value}`);
-                break;
             case 'dark-theme':
                 root = this.widget.get_root();
                 if(!root) break;