[ie/niconico] remove unusable legacy danmaku (comments, subtitle) extraction

[pzhlkj6612]

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Description of your pull request and other information

Hi.

Fixes #8684

See also: #6773

CAUTION: This PR completely removes the code about legacy danmaku extraction.

The "new danmaku extraction" logic works fine, by the way.

Test

It seems that all /comment/threads/.../server are empty strings, I guess the legacy endpoint has been shut down.

Here is the api_data of sm16309076 - 「Thousand Cherry Blossoms」 Played it【Piano】 - ニコニコ動画:

{
  ...
  "comment": {
    "server": {
      "url": ""
    },
    "keys": {
      "userKey": ""
    },
    "layers": [
      {
        "index": 0,
        "isTranslucent": false,
        "threadIds": [
          {
            "id": 1322648373,
            "fork": 1,
            "forkLabel": "owner"
          }
        ]
      },
      {
        "index": 1,
        "isTranslucent": false,
        "threadIds": [
          {
            "id": 1322648373,
            "fork": 0,
            "forkLabel": "main"
          },
          {
            "id": 1322648373,
            "fork": 2,
            "forkLabel": "easy"
          }
        ]
      }
    ],
    "threads": [
      {
        "id": 1322648373,
        "fork": 1,
        "forkLabel": "owner",
        "videoId": "sm16309076",
        "isActive": false,
        "isDefaultPostTarget": false,
        "isEasyCommentPostTarget": false,
        "isLeafRequired": false,
        "isOwnerThread": true,
        "isThreadkeyRequired": false,
        "threadkey": null,
        "is184Forced": false,
        "hasNicoscript": true,
        "label": "owner",
        "postkeyStatus": 0,
        "server": ""                         <-----------------
      },
      {
        "id": 1322648373,
        "fork": 0,
        "forkLabel": "main",
        "videoId": "sm16309076",
        "isActive": true,
        "isDefaultPostTarget": true,
        "isEasyCommentPostTarget": false,
        "isLeafRequired": true,
        "isOwnerThread": false,
        "isThreadkeyRequired": false,
        "threadkey": null,
        "is184Forced": false,
        "hasNicoscript": false,
        "label": "default",
        "postkeyStatus": 0,
        "server": ""                         <-----------------
      },
      {
        "id": 1322648373,
        "fork": 2,
        "forkLabel": "easy",
        "videoId": "sm16309076",
        "isActive": true,
        "isDefaultPostTarget": false,
        "isEasyCommentPostTarget": true,
        "isLeafRequired": true,
        "isOwnerThread": false,
        "isThreadkeyRequired": false,
        "threadkey": null,
        "is184Forced": false,
        "hasNicoscript": false,
        "label": "easy",
        "postkeyStatus": 0,
        "server": ""                         <-----------------
      }
    ],
    "ng": {
      "ngScore": {
        "isDisabled": false
      },
      "channel": [],
      "owner": [],
      "viewer": {
        "revision": 1,
        "count": 0,
        "items": []
      }
    },
    "isAttentionRequired": false,
    "nvComment": {
      "threadKey": "eyJ0...",
      "server": "https://nv-comment.nicovideo.jp",
      "params": {
        "targets": [
          {
            "id": "1322648373",
            "fork": "owner"
          },
          {
            "id": "1322648373",
            "fork": "main"
          }
        ],
        "language": "en-us"
      }
    }
  },
  ...

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