Remove deprecated where clause in generic arguments once apple/swift/test no longer uses language mode 4

[ahoppen]

Once apple/swift/test/ defaults to language mode 5, rather than language mode 4, we should be able to remove the deprecated where clause in generic arguments. I already had a PR up for this here but closed it because we can’t merge it yet. swiftlang/swift#63602

[ahoppen]

rdar://106865011

[ahoppen]

swift-syntax supports parsing in the Swift 4 language mode with #2466 and we thus need to continue supporting it.