Remove signature helper completely from Invidious (#5550)

* Remove signature helper completely from Invidious

The official way to reproduce video with Invidious now is by using
Invidious Companion which uses Youtube.JS with a Javascript Interpreter
that can successfully decrypt youtube video URLs.

Sig helper has not been used for a long time, is beyond broken and no
one has plans to fix it and maintain it.

* Remove DECRYPT_FUNCTION and shrink player function

* remove `sp = cfr[sp]`

* Improve message

src/invidious/videos.cr

@@ -326,6 +326,14 @@ end
 def fetch_video(id, region)
   info = extract_video_info(video_id: id)
 
+  if info.nil?
+    raise InfoException.new("Invidious companion is not available. \
+    Video playback cannot continue. \
+    If you are the administrator of this instance, install Invidious companion \
+    following the installation instructions \
+    <a href=\"https://docs.invidious.io/installation/\">https://docs.invidious.io/installation/</a>")
+  end
+
   if reason = info["reason"]?
     if reason == "Video unavailable"
       raise NotFoundException.new(reason.as_s || "")