Remove use of wipefs to clear file system signatures (#688882)

The wipefs command has the following significant limitations which were
worked around in previous commits:

1)  Wasn't available in the earliest distributions supported by GParted;

2)  Had to be called 3 times to erase vfat (fat16/32) signatures in all
    but the most recent versions.

This meant we had all the code to clear file system signatures without
using the wipefs command as well as extra complexity of using wipefs
too.  So just remove use of the wipefs command.

Bug #688882 - Improve clearing of file system signatures