Cannot disable Jira issue list when other trackers have been enabled

Summary

When the Jira issue list feature is configured and enabled and another trackers is activated afterwards, it is not possible to disable the Jira issue list.

Steps to reproduce

1. Setup the Jira integration for a project and enable View Jira issues in GitLab
2. Go to and setup the Assembla integration
3. Go back to the Jira Integration and try to disable View Jira issues in GitLab

Example Project

What is the current bug behavior?

What is the expected correct behavior?

1. As an admin setting up integrations, I would expected to be able to disable this feature, even when other trackers are enabled
2. I would expect to not get into this situation by seeing a message on the Assembla integration (or any other third-party tracker) that would tell me that I can not enable any new trackers without disabling the old one first.

Relevant logs and/or screenshots

Error message when trying to disable Jira issue list

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes