Remove hack to request VS Code insiders when running tests via `vscode-test`

[brettcannon]

microsoft/vscode-test#202 updated vscode-test so that you don't need to be so careful to request the latest VS Code insiders without a version number.

[karrtikr]

Looks like we missed to verify it, #21180 seems to have broken ability to specify insider engines. Reverting the change and reopening.

cc/ @paulacamargo25