`platform` option is missing

[SeanTAllen]

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Once upon a time, we had a --platform option to set the platform to install for. At some point, it was mostly removed except in Main where we attempt to look at the option (which isn't in the spec so can't be set) and otherwise we check the .platform file to determine the platform OR exit.

So, at the moment you have to have a .platform file. This might not be the right thing to do, but we need to look at how all this would come together if we add --platform back in.

[SeanTAllen]

We discussed during sync and decided that we can't come up with a use case that makes --platform valuable so the currently "useless" code should be removed and .platform should be used instead.

if .platform is missing then default command could be used.

there's no bug here, it's "just" some cleanup".