Revert "scx_rusty: Refactor ridx assignment in populate_tasks_by_load"

[Byte-Lab]

Reverts #382

Causes the following issue:

21:32:36 [INFO] Running scx_rusty (build ID: 0.8.1-g41d60aef04d2a7fc0da738bf50391ea5f232ddd3 x86_64-unknown-linux-gnu/debug)                                
21:32:36 [INFO] ---                                                           
21:32:36 [INFO] NUMA[00] mask= 0b11111111111111111111111111111111             
21:32:36 [INFO]   DOM[00] mask= 0b00000000111111110000000011111111            
21:32:36 [INFO]   DOM[01] mask= 0b11111111000000001111111100000000            
21:32:36 [INFO] Rusty scheduler started!                                                                                                                    
thread 'main' panicked at src/load_balance.rs:689:25:
attempt to subtract with overflow
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

We can instead add a comment explaining the weird logic, or figure out another way to do it.