Simplify USB storage handling

[Short-bus]

The 'discmonitor' class in src/pilomardisc.py tries to find and mount USB storage if the RPi is running without the desktop being launched.
If the desktop is automatically launched in the background when the RPi boots (raspi-config option) then the USB storage mounts automatically and more reliably.
The code in discmonitor can therefore be simplified and more reliable if the decision is made to always have the desktop launched on startup if you want to use attached storage.