Remove special deprecated issue route code added to issues/_discussion.html.haml

Problem

!24791 (merged) introduced some code into issues/_discussion.html.haml to support both the legacy issue route and the new \-\ scoped issue route !24791 (comment 295834850).

In %13.0 the legacy issue routes will redirect to the new \-\ routes, and so the view file will no longer need to support legacy routes.

Solution

Remove the support for the legacy route #208669 (comment 296879206).

Original description:

The following discussion from !24791 (merged) should be addressed:

• @engwan started a discussion: (+5 comments)

  @dzaporozhets code here looks good to me but I tested locally and this breaks the "Designs" tab in issues when accessing the issue via the route without the -.

  This is similar to #202049 (closed)

  createRouter in https://gitlab.com/gitlab-org/gitlab/blob/43976583aa563ce44f765935cb775fdb54308877/ee/app/assets/javascripts/design_management/index.js#L13 is using the issue path with - as the base so when accessing it without - the Vue router doesn't work properly.