Remove custom CORS controller for JiraConnect

What does this MR do and why?

This MR fixes a bug where JiraConnect controllers don't send the correct CORS headers.

We want the Access-Control-Allow-Origin header to match the jira_connect_proxy_url application setting. Only the configured URL is the allowed origin. I thought this wasn't possible using rack cors because origins are fixed values that are loaded at the application startup. Therefore, I created a custom solution that routes the OPTIONS requests to a controller, handling the headers.

It turned out that the solution never worked because rack cors intercepts all options requests before they reach the controller. I now found that it's possible to pass a block to the origins method that solves the problem in a more elegant way and fixes the bug.

How to set up and validate locally

1. Switch to the master branch
2. Set the jira_connect_proxy_url setting.

   Gitlab::CurrentSettings.update(jira_connect_proxy_url: 'http://example.com')

3. Check the response, headers of an OPTIONS request to one of the jira_connect endpoints.

   curl -v --location --request OPTIONS 'http://127.0.0.1:3000/-/jira_connect/oauth_application_id' \
   --header 'access-control-request-method:  GET' \
   --header 'Origin: http://example.com'

4. The headers should not include the Access-Control-Allow-Origin header.
5. Switch to the andysoiron/fix-jira_connect-cors-problems branch and restart GDK to pick up the new settings.
6. Check the response, headers of an OPTIONS request to one of the jira_connect endpoints.

   curl -v --location --request OPTIONS 'http://127.0.0.1:3000/-/jira_connect/oauth_application_id' \
   --header 'access-control-request-method:  GET' \
   --header 'Origin: http://example.com'

7. The response headers should now include the Access-Control-Allow-Origin header with the value of http://example.com.
8. Do the request again with another origin.
9. Check the response, headers of an OPTIONS request to one of the jira_connect endpoints.

   curl -v --location --request OPTIONS 'http://127.0.0.1:3000/-/jira_connect/oauth_application_id' \
   --header 'access-control-request-method:  GET' \
   --header 'Origin: http://some_other_origin.com'

10. The response headers should not include an Access-Control-Allow-Origin header.

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