d/rules: remove --parallel from dh invocation

 - Each c++ process might take up to 2.5 GB of RAM. Arm buildds only
   have 4 GB of RAM but build with DEB_BUILD_OPTIONS=parallel=5. This
   means that the build might consume 12 GB of RAM at a time which brings
   the buildd to a standstill as 8 GB have to be swapped out.
 - This change is to try out whether it solves the FTBFS bug #769864 on
   armhf.