Remove unused icons

src/vs/workbench/contrib/files/browser/media/CollapseAll.svg

-<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="-1 0 16 16" enable-background="new -1 0 16 16"><path fill="#424242" d="M14 1v9h-1v-8h-8v-1h9zm-11 2v1h8v8h1v-9h-9zm7 2v9h-9v-9h9zm-2 2h-5v5h5v-5z"/><rect x="4" y="9" fill="#00539C" width="3" height="1"/></svg>
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src/vs/workbench/contrib/files/browser/media/CollapseAll_inverse.svg

-<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="-1 0 16 16" enable-background="new -1 0 16 16"><path fill="#C5C5C5" d="M14 1v9h-1v-8h-8v-1h9zm-11 2v1h8v8h1v-9h-9zm7 2v9h-9v-9h9zm-2 2h-5v5h5v-5z"/><rect x="4" y="9" fill="#75BEFF" width="3" height="1"/></svg>
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