Remove Jira Issue username

We are not going to use the username because Jira API doesn't provide
`name` in the case of Jira Cloud.

This partially reverts 63082f24.

ee/app/serializers/integrations/jira/issue_entity.rb

@@ -79,7 +79,6 @@ module Integrations
       def jira_user(user)
         {
           name: user['displayName'],
-          username: user['name'],
           web_url: jira_web_url(user),
           avatar_url: user['avatarUrls']['48x48']
         }

ee/spec/serializers/integrations/jira/issue_detail_entity_spec.rb

@@ -87,13 +87,11 @@ RSpec.describe Integrations::Jira::IssueDetailEntity do
       ],
       author: hash_including(
         name: 'reporter',
-        username: 'reporter@reporter.com',
         avatar_url: 'http://reporter.avatar'
       ),
       assignees: [
         hash_including(
           name: 'assignee',
-          username: 'assignee@assignee.com',
           avatar_url: 'http://assignee.avatar'
         )
       ],
@@ -103,11 +101,10 @@ RSpec.describe Integrations::Jira::IssueDetailEntity do
       comments: [
         hash_including(
           id: '10022',
-          author: hash_including({
+          author: hash_including(
             name: 'comment_author',
-            username: 'comment@author.com',
             avatar_url: 'http://comment_author.avatar'
-          }),
+          ),
           body_html: "<p dir=\"auto\">Comment</p>",
           created_at: '2020-06-25T15:50:00.000+0000'.to_datetime.utc,
           updated_at: '2020-06-25T15:51:00.000+0000'.to_datetime.utc

ee/spec/serializers/integrations/jira/issue_entity_spec.rb

@@ -61,13 +61,11 @@ RSpec.describe Integrations::Jira::IssueEntity do
       ],
       author: hash_including(
         name: 'reporter',
-        username: 'reporter@reporter.com',
         avatar_url: 'http://reporter.avatar'
       ),
       assignees: [
         hash_including(
           name: 'assignee',
-          username: 'assignee@assignee.com',
           avatar_url: 'http://assignee.avatar'
         )
       ],