[plugin] cleanup workaround for FileManager.copyItem on Linux

[sebsto]

When swift-foundation/issues#1125 will be fixed, cleanup the code that introduced a temporary workaround

Original PR : #467
Original Issue : #449
Lines of code to remove : https://github.com/swift-server/swift-aws-lambda-runtime/blob/71d49b049ec1325ae064661376e586577923555a/Plugins/AWSLambdaPackager/Plugin.swift#L258

Upstream issue : swiftlang/swift-foundation#1125