Resolve broken imports on Windows

Commit b5affca26b8a65a54dde7271de4479b6f01db595 removes support for GHC versions less than 7.0, but it looks like `Network.Socket.ByteString` still attempts to import `readRawBufferPtr` and `writeRawBufferPtr` from `GHC.Handle` on Windows, even though it's now located in `GHC.IO.FD`. This commit simply removes the `GHC.Handle` import so that `network` will build correctly on Windows.