CLEANUP: haload: embed rate_task into hld_thr_info structure

CLEANUP: haload: embed rate_task into hld_thr_info structure

Store the rate task directly inside the per-thread <hld_thr_info>
struct instead of keeping a separate global <hld_rate_tasks> array.

This simplifies memory management by grouping thread-specific tasks and
data together:

  - move rate task creation into <hld_alloc_thrs_info>
  - clean up rate tasks directly inside <hld_dealloc_thrs_info>
  - remove the separate <hld_dealloc_rate_task> helper and global array