[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 :

swift-aws-lambda-runtime/Plugins/AWSLambdaPackager/Plugin.swift

Line 258 in 71d49b0

} catch let error as CocoaError {

Upstream issue : swiftlang/swift-foundation#1125