DLSCH scheduler optimization

Summary

We have done some minor optimization and code cleaning for the default ULSCH scheduler and we want to do the same for the default DLSCH scheduler. Again, we saw many small optimizations and code cleaning that can be done. This job is still done in the context of CDRX implementation.

It concerns (non exhaustiv):

• eNB_scheduler_dlsch.c
• eNB_scheduler_primitives.c
• pre_processor.c

We will do regular commit to check that our modifications doean't break the code and is compatible with the CI test.