test: dm: usb: Update test cases for USB

test: dm: usb: Update test cases for USB

Now that we have changed to remove all devices under the root hub in
usb_stop(), and corrected the USB emulator select logic, it makes no
sense to do various tests based on 'usb tree' output since the order
of devices is no longer fixed. Remove these USB test cases related
to 'usb tree'.

For the USB remove test, ideally we should remove an emulator device
node from the device tree, but this is so far not working. Change to
test the 'usb stop' only.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>