staging: btmtk_usb: remove driver

No one seems to be working on it anymore, and it really should be merged
into the already-existing btusb driver.  Also, there is not any proper
author attribution on the code (it was copied from the in-kernel
driver...)

If someone wants to pick this back up, we can easily revert this, but
for now, delete the driver.

Cc: Yu-Chen, Cho <acho@suse.com>
Cc: Jay Hung <jay.hung@mediatek.com>
Cc: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>