qemu-iotests: Remove PERL_PROG and BC_PROG

We test for the presence of perl and bc and save their path in the
variables PERL_PROG and BC_PROG, but never actually make use of them.
Remove the checks and assignments so qemu-iotests can run even when
bc isn't installed.

Reported-by: Yash Mankad <ymankad@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
