Remove "forest.css" in Mermaid

[Gowa2017]

在新版本的Mermaid中已经不需要。而加上这个css文件会影响graphviz里面node的样式

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This is unnecessary in new Mermaid. This CSS file will affect node style in Graphviz.

Closes #1138

[FranklinYu]

Both the method mermaidStylesheets and the file mermaid.forest.css were introduced in #865. The CSS file looks like a theme. According to unpkg, Mermaid team stop providing those theme files (including “forest”, “neutral”, and “dark”) since 7.0.11 release. Do you know what’s the current way to apply the forest theme?

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你知道最新的Mermaid怎么改主题么？最好能保持使用Forest主题。

[Gowa2017]

From the mermaid official document, http://mermaid-js.github.io/mermaid/#/mermaidAPI?id=mermaidapi-configuration-defaults, in version 8, we can use mermaid.initialize(config); to setting the mermaid, like

var config = {
    theme:'default',
    logLevel:'fatal',
    securityLevel:'strict',
    startOnLoad:true
}
mermaid.initialize(config);

theme can be http://mermaid-js.github.io/mermaid/#/mermaidAPI?id=theme.

[FranklinYu]

That's nice. I would prefer to do all related things in a single PR.

1. Remove the related part in MPRenderer.m (instead of only commenting it out). We can easily revert the change if we regret. That's the point for Git.
2. Remove the Mermaid style sheet.
3. Update MacDown/Resources/Extensions/mermaid.init.js accordingly.

[Gowa2017]

#1165

Now, please close this issue