sync v2: finish total sync-chain removal feature

[bradleyrichter]

As a user, I need to be able completely remove a sync chain from any device where all devices in the chain will be removed after a confirmation dialog.

The device that executes the command will update the sync database to remove all files. Each individual device will need to recognize that they were removed and update accordingly. This includes a notification message to the user when they next poll the sync server, or push new data.

[cezaraugusto]

how this differs from current "reset sync" functionality?

[NejcZdovc]

@cezaraugusto I think this feature removes data from other devices as well, where reset removes it only from this device

[bsclifton]

Closing as wontfix since we're going to switch gears and focus more on Brave Core. This is captured there with brave/brave-browser#356