Simplify deleting partitions in libtest

Simplify deleting partitions in libtest

Eliminate unnecessary code duplication. We can use a for-loop instead
of a while-loop. There is no need to echo $DISKSARRAY in a subshell or
return 0. Declare all variables with typeset.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9224