Remove {{warning}} macro from translated-content

[yin1999]

The {{warning}} macro has been deprecated, let's remove it from translated-content.

This macro will render a note card, we can easily replace it by:

HTML:

<div class="notecard warning">
  <p><strong>{{warning-keyword}}</strong> {{warns}}</p>
</div>

Markdown:

> **{{warning-keyword}}** {{warns}}

The {{warning-keyword}} could be found here (card_warning_label.msgstr field). And the {{warns}} is just the argument of {{warning}} macro.

An example in mdn/content (mdn/content#20015).

[mfuji09]

Removed from ja.