rtw89: remove cch_by_bw which is not used

Originally, cch_by_bw recorded center channels of each available
bandwidths under current bandwidth. And the plan was to iterate
cch_by_bw as parameters to query other configurations. However,
we have not used it for the time being. Keeping it will disturb
the follow-up things, such as bandwidth 160 MHz, so we remove it
for now. If it's really needed at some point, we will redesign it.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20211201080901.12125-1-pkshih@realtek.com