virsh_net_create: Remove incorrect redundant test

Remove the 'variant' for 'existing_network' within the variants for
bad_command_line.  Since each of the error_test variants already has
a 'no_existing_removal' and 'existing_removal' option, the removed
code would was redundant. Additionally, since the existing_network
option didn't have a net_create_options_ref, net_create_options_extra,
or net_create_corrupt_xml parameter, the test went through the high
level test code which was not expected for error_test code unless
the net_create_remove_existing is set to yes.
Merge pull request #960 from jferlan/virsh_net_create

virsh_net_create: Remove incorrect redundant test