Remove the "Preview" and "Write" tabs when editing the markdown files in single-file editor

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After the "Preview" tab on the markdown files is connected to the live-preview functionality and the toolbar is globally available, we don't need the tab anymore.

At the same time, after removing that tab, the "Write" tab will be the only tab on the markdown files. So it makes sense to remove it as well to simplify the UI.

Before	After