Remove ICommandAction#iconClassName

[jrieken]

This got added via c9f9081 and is a no, no because it a) weirdly competes with iconLocation give consumer headaches what to use and b) won't work for the Mac Touch Bar (which is the reason this originally has been added). In essence this must go again and we should use uri until it comes to rendering

[jrieken]

fyi - with #85635 we are adding support to have uris like this vscode-icon://codicon/XYZ That means codicon can then be used everywhere, just as uris.

[jrieken]

Adding @eamodio since the removal could be a side effect of #85635

[jrieken]

I am preparing a change for #84695 and I am cleaning this up along the way. A command icon is now defined like so: icon?: { dark?: URI; light?: URI; } | ThemeIcon;, a sample for codicons is icon: { id: "codicon.go-to-file" }

[miguelsolorio]

@jrieken thanks for cleaning this up (and sorry for introducing this the wrong way)!