libparted: remove has_partitions check to allow loopback partitions

libparted: remove has_partitions check to allow loopback partitions

Commit 1b766b69 added the _has_partitions function to check if a device
was a loop device and if so, skip updating the kernel partition table
because loop devices did not support partitions.  This function never
worked anyway, and loop devices now (since linux 3.2) support
partitions, so remove this crufty code.
* libparted/arch/linux.c (_has_partitions): Remove function and
all uses.