Remove MAKE_FLAGS from CI variables

MAKE_FLAGS as environment variables still applied although they were not
set in the 'dunecontrol' command

.gitlab-ci.yml

@@ -10,8 +10,6 @@ variables:
   CMAKE_FLAGS:
     -DDUNE_PYTHON_VIRTUALENV_SETUP=True
     -DDUNE_PYTHON_ALLOW_GET_PIP=True
-  MAKE_FLAGS:
-    -j $CPUS_MULTICORE
 
 image: $DUNE_ENV_IMAGE