Remove Linux kernel driver for plustek_pp backend

The plustek_pp backend includes source code for an out-of-tree Linux kernel driver pt_drv.ko, which can be built and utilized by the backend.

With the changes in !521 (merged) the kernel driver no longer builds. The plustek_pp backend can be configured to access the parallel port directly though.

It would seem best to just remove the kernel driver and the code which interfaces with it.