disk: remove USB MSC specific FAT12 ramdisk

For USB mass storage class, a fixed size 16 KiB FAT12 ramdisk
is forced. This is not really visible to the user and
the DISK_RAM_VOLUME_SIZE option is ignored.
There is no use of such a small file system and for USB MSC
testing, like throughput or USB3CV, a file system is not necessary.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>