[Swiftify] remove SwiftifyProtocolInfoPrinter (NFC)
This class is unused since safe interop wrapper support for ObjC methods
was backed out. When we reintroduce this, it will be implemented
differently, and this class won't be necessary - at least in its current
reincarnation. Move as much as possible into swiftifyImpl, making
swiftify a thin wrapper around it. This thin wrapper will be relevant
when we reimplement support for ObjC methods.
