Remove unused eslint-plugin-html

With a quick check it was easily discoverable that the plugin is
currently not used.

```
node_modules/.bin/eslint --print-config app/ | grep html
```

It has been added with 8050d835 but removed with eb336d71 for vue
files and completely removed with 3e9bb4ad.

As this plugin is unused and actively hindering an eslint upgrade, we
should remove it.

.eslintrc.yml

@@ -8,7 +8,6 @@ globals:
   IS_EE: false
 plugins:
   - import
-  - html
   - "@gitlab/i18n"
   - "@gitlab/vue-i18n"
 settings:

package.json

@@ -163,7 +163,6 @@
     "eslint": "~5.9.0",
     "eslint-import-resolver-jest": "^2.1.1",
     "eslint-import-resolver-webpack": "^0.10.1",
-    "eslint-plugin-html": "5.0.0",
     "eslint-plugin-import": "^2.14.0",
     "eslint-plugin-jasmine": "^2.10.1",
     "eslint-plugin-jest": "^22.3.0",

yarn.lock

@@ -4631,13 +4631,6 @@ eslint-plugin-filenames@^1.3.2:
     lodash.snakecase "4.1.1"
     lodash.upperfirst "4.3.1"
 
-eslint-plugin-html@5.0.0:
-  version "5.0.0"
-  resolved "https://registry.yarnpkg.com/eslint-plugin-html/-/eslint-plugin-html-5.0.0.tgz#396e30a60dedee0122fe08f11d13c5ab22f20d32"
-  integrity sha512-f7p/7YQdgQUFVAX3nB4dnMQbrDeTalcA01PDhuvTLk0ZadCwM4Pb+639SRuqEf1zMkIxckLY+ckCr0hVP5zl6A==
-  dependencies:
-    htmlparser2 "^3.10.0"
-
 eslint-plugin-import@^2.14.0, eslint-plugin-import@^2.16.0:
   version "2.16.0"
   resolved "https://registry.yarnpkg.com/eslint-plugin-import/-/eslint-plugin-import-2.16.0.tgz#97ac3e75d0791c4fac0e15ef388510217be7f66f"