remote_access: make external usage a hard error

Nothing should be using this anymore, so make it a hard failure.

BUG=b:191307774
TEST=CQ passes

Change-Id: I2ad075a2bfecab6c6b58e10c29bd9d8f4b4c0862
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/4300649
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Sergey Frolov <sfrolov@google.com>
Tested-by: Mike Frysinger <vapier@chromium.org>