Remove support for macOS

[unnamedd]

As soon the initializer Image(systemName:) doesn't have support to macOS yet, in my point of view, doesn't make sense to support the platform yet.

Checking SwiftUI headers, specifically for the init method mentioned above, macOS/OSX is currently unavailable, for this reason I'm creating this pull request.

Here the snippet of code extracted from SwiftUI headers:

@available(iOS 13.0, tvOS 13.0, watchOS 6.0, *)
@available(OSX, unavailable)
extension Image {

    @available(OSX, unavailable)
    public init(systemName: String)
}

[fredpi]

Thanks @unnamedd for noticing and fixing this! 👍