SNPowerShell is only for Windows, remove OS check

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What does this MR do?

It removes OS check when testing for shell powershell
This MR is a cleanup before !5443 (closed). It doesn't go as far as reverting commit 5aa41d29 because that may be too much.

Why was this MR needed?

!5103 (merged) introduced a shell and OS check to apply special quoting during GetGitCredHelperCommand, which led to !5384 (merged)

However, since PowerShell (Desktop) 5.1 is only available for Windows, checking the OS was not necessary.

What's the best way to test this MR?

See !5384 (merged)

What are the relevant issue numbers?