remove redundant setting of default volume/backingStore mode

According to the docs, for volumes and backingStores: "The mode defaults to
0600 when not provided.  https://libvirt.org/formatstorage.html

Removing this setting here allows the pool configuration to set the default
mode.

lib/vagrant-libvirt/action/create_domain_volume.rb

@@ -51,7 +51,6 @@ module VagrantPlugins
                   xml.permissions do
                     xml.owner storage_uid(env)
                     xml.group storage_gid(env)
-                    xml.mode '0600'
                     xml.label 'virt_image_t'
                   end
                 end
@@ -61,7 +60,6 @@ module VagrantPlugins
                   xml.permissions do
                     xml.owner storage_uid(env)
                     xml.group storage_gid(env)
-                    xml.mode '0600'
                     xml.label 'virt_image_t'
                   end
                 end