Removed characters from Mock.swifttemplate since it's obsolete in Swift 5.0

[davidetamiazzo]

To fix the compilation issues that arise from using the file Mock.swifttemplate in Swift 5.0, I have proceeded with removing the property .characters from the file.

With this property, we get the following kind of errors:

/private/var/folders/21/8dtt1gkn5175rwk038fjbv300000gp/T/SwiftTemplate/2.1.3/Sources/SwiftTemplate/main.swift:1192:21: error: 'characters' is unavailable: Please use String directly
        return part.characters.split(separator: ":").map(String.init).first!
                    ^~~~~~~~~~
Swift.String:5:16: note: 'characters' was obsoleted in Swift 5.0
    public var characters: String { get set }
               ^
error: fatalError

String doesn't have the property characters anymore: https://developer.apple.com/documentation/swift/string/1540072-characters

[markst]

Is this a dupe of https://github.com/MakeAWishFoundation/SwiftyMocky/pull/352 ?