Remove onehot2bin/bin2onehot encoders in br_arb_rr

[zhemao-openai]

The round robin arbiter stores the last grant used to mark the priority as a binary-encoded signal. This means the timing path has to go through a bin2onehot encoder and then back through onehot2bin encoder before getting updated. We should change to saving the onehot-encoded priority state instead of improve timing.

[zhemao-openai]

Fixed by #216