Fix build with newer versions of mono

Current HEAD builds with mono version 2.10.8.1, but fails with version 3.2.6 (and probably earlier versions):

Protocol/MessageReader.cs(579,25): error CS0208: Cannot take the address of, get the size of, or declare a pointer to a managed type `T'
Protocol/MessageWriter.cs(431,26): error CS0208: Cannot take the address of, get the size of, or declare a pointer to a managed type `T'
make[1]: *** [dbus-sharp.dll] Error 1
make[1]: Leaving directory `/var/lib/jenkins/customws/mono-master-stack/src/dbus-sharp2/src'
make: *** [all-recursive] Error 1

This PR reverts the commits that I identified as the culprits.