Only attempt removal of the rule allowing tftp if it was added

Only attempt removal of the rule allowing tftp if it was added

During virtual network startup, the iptables rule that allows tftp
traffic is only added if network->def->tftproot is non-empty, but when
the virtual network is destroyed, we had been unconditionally trying
to delete the rule. This was harmless, except that it created a bogus
error message.

This patch conditionalizes the delete command in the same manner that
the insert command is already conditionalized.