bugfix: cpu loads are not computed, hence it makes no sense to target CPUs for load balancing

[abouteiller]

In some cases CPU SYRKs would execute when there is a valid GPU hook, because the CPU load (which is never updated hence is always having an eta of 0+time_estimate) would be lower than the data AREAD preferred GPU load+time_estimate.

Since the CPU loads are always 0 this is not accurate, and we should load balance only between GPUs.