Remove nw1 mode

[rogerwang]

After nw2 mode released as default (#5875), it will get stablized in several major versions. Then the code for nw1 mode will be removed to save rebase effort. This issue is for tracking nw1 removal.

[rogerwang]

If anyone found their issue in nw2 and think it's blocking nw1 removal, please file an issue for nw2 and CC this issue.