USB ask mode is basically a footgun: it can't work on native targets
and doesn't work reliably on hosted ones, and it continually produces
a slow trickle of problems. FS#13317 gives a rundown of the issues.
Removing the setting seems like the best solution for now, since a fix
would be pretty involved.
This partially reverts 60f581e8f5. The USB Mode setting is left in
place so the option can be added back later in a non-buggy way.
Change-Id: Ie01b28dd2ed95a31b509a7834d85bac8eb866098
7e0492444c