[cpp-electron] clean no longer useful @theia/cpp preferences

[marcdumais-work]

I noticed we still have what seems to be preferences to @theia/cpp, which we no longer use. These can probably be removed, unless we think some vscode-clangd preferences would be good to have instead.

https://github.com/theia-ide/theia-apps/blob/master/theia-cpp-electron/package.json#L26-L28

[stale]

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