bug#61497: emacs-lsp-treemacs unused leftover icons in sources

[Jelle Licht <jlicht@fsfe.org>]

Commit e0d2ec418bb on master removed icons that are unclearly licensed
from the sources of emacs-lsp-treemacs. Quoted here: 

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gnu: emacs-lsp-treemacs: Remove unclearly licensed icons.

emacs-lsp-treemacs bundles icons with unclear licenses.
See also <https://github.com/emacs-lsp/lsp-treemacs/issues/123>.
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Some icons are still left in the sources, in the 'icons/vscode'
directory' of the source tarball one builds by running `guix build
--source emacs-lsp-treemacs'. I have never used vscode, and am
unfamiliar with the licensing situation of it and its related icons.

In case these icons are also unclearly licensed, I propose we follow the
same strategy as done earlier by Liliana, and remove the vscode icons
entirely.  If the icons are actually licensed under an unproblematic
license, it would be nice if they were installed when running `guix
install emacs-lsp-treemacs', and the license property of the package
updated to reflect this fact.

Thanks for any insight,
- Jelle