Unable to delete unused VLAN

[Nick Niehoff]

After creating a VLAN in the UI the user is unable to delete the same VLAN.

Steps to reproduce:

1. Navigate to the subnets tab of MAAS 2.6
2. Add a VLAN with any vlan id
3. Attempt to delete the VLAN via the UI

Workaround via CLI:

export FABRIC_NAME=TheNameOfTheFabric
export VLAN_ID=123

FABRIC_ID=$(maas admin fabrics read | jq --arg name ${FABRIC_NAME} '.[] | select(.name == $name) | .id')
maas admin vlan delete $FABRIC_ID $VLAN_ID

[Heather Lemon (hypothetical-lemon)]

screenshot of error

[Heather Lemon (hypothetical-lemon)]

this seems to work on the 2.7 and 2.8 branch, just an issue with 2.6