Delete unused icons from DevTools

Our high def resources are not being selected. We use `foo@2x.png` in the asset name - which is the IntelliJ convention - but we should be using `.../2.0/foo.png`, which is the flutter convention.

This can likely be address with a quick command-line script and some regexs through the codebase.

See https://github.com/flutter/devtools/pull/2069#pullrequestreview-429012584 for context.

首先在代码库中搜索 @2x.png 资源名称以及高分辨率资源选择逻辑。查看关联 pull request 的上下文，然后验证受影响的图标是否使用 2.0 目录约定，以及是否正确选择了高分辨率资源。

由索引模型根据 Issue 内容生成。